Britain will soon get “slapped in the face” for last week's capture of an Iranian supertanker, a cleric was quoted as saying Friday amid rising tensions between the two nations in the Gulf.
Cleric Kazem Sedghi, an adviser to Iran's Supreme Leader Ayatollah Ali Khamenei, told worshipers during Tehran’s Friday prayer sermon broadcast live on Iranian state TV that the United Kingdom should be worried for their actions off the coast of Gibraltar.
“Iran’s strong establishment will soon slap Britain in the face for daring to seize the Iranian oil tanker,” he said.
Sedghi’s warning came after the Iranian government on Friday called Britain to immediately release the oil tanker that British Royal Marines seized last week on suspicious it was breaking European sanctions by taking oil to Syria.
Iran’s Foreign Ministry spokesman accused London of playing a “dangerous game” a day after police in Gibraltar, a British overseas territory on the southern tip of Spain, said they arrested the captain and chief officer of the supertanker.
Abbs Mousavi told Iranian state news agency IRNA that “the legal pretexts for the capture are not valid … the release of the tanker is in all countries’ interest.”
“This is a dangerous game and has consequences,” he added.
Gibraltar has insisted its decision to detain the Iranian tanker was taken alone and not on orders from any government, despite a senior Spanish official previously saying the interception was carried out at the request of the United States.
“All relevant decisions in respect of this matter were taken only as a direct result of the government of Gibraltar having reasonable grounds to believe the vessel was acting in breach of established E.U. sanctions against Syria,” Fabian Picardo, the territory’s chief minister, told reporters. “There has been no political request at any time from any government that Gibraltar should act or not act on one basis or another.”
The detained vessel contained 2.1 million barrels of light crude oil, he added.
The British navy said Thursday it had stopped three Iranian paramilitary vessels from disrupting the passage of a British oil tanker through the Strait of Hormuz, the narrow mouth of the Persian Gulf. The brief but tense standoff stemmed from the U.K.'s role in seizing the Iranian tanker.
In this image from file video provided by UK Ministry of Defence, British navy vessel HMS Montrose escorts another ship during a mission to remove chemical weapons from Syria at sea off the coast of Cyprus in February 2014. Five Iranian Islamic Revolutionary Guard Corps gunboats tried to seize a British oil tanker in the Strait of Hormuz Wednesday but backed off after a British warship approached, a senior U.S. defense official told Fox News.
(AP/UK Ministry of Defence)
On Friday, the British Ministry of Defense said it was moving up its timetable for relieving the HMS Montrose, a frigate operating in the Persian Gulf, with the larger HMS Duncan destroyer in the wake of the recent developments.
"This will ensure that the UK alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping lane."
Iran recently began surpassing uranium enrichment limits set in its 2015 nuclear deal with world powers in response to President Donald Trump's decision to pull the U.S. out of the accord a year ago. He also has re-imposed tough sanctions on Tehran's oil exports, exacerbating an economic crisis that has sent its currency plummeting.
Britain will soon get “slapped in the face” for last week's capture of an Iranian supertanker, a cleric was quoted as saying Friday amid rising tensions between the two nations in the Gulf.
Cleric Kazem Sedghi, an adviser to Iran's Supreme Leader Ayatollah Ali Khamenei, told worshipers during Tehran’s Friday prayer sermon broadcast live on Iranian state TV that the United Kingdom should be worried for their actions off the coast of Gibraltar.
“Iran’s strong establishment will soon slap Britain in the face for daring to seize the Iranian oil tanker,” he said.
Sedghi’s warning came after the Iranian government on Friday called Britain to immediately release the oil tanker that British Royal Marines seized last week on suspicious it was breaking European sanctions by taking oil to Syria.
Iran’s Foreign Ministry spokesman accused London of playing a “dangerous game” a day after police in Gibraltar, a British overseas territory on the southern tip of Spain, said they arrested the captain and chief officer of the supertanker.
Abbs Mousavi told Iranian state news agency IRNA that “the legal pretexts for the capture are not valid … the release of the tanker is in all countries’ interest.”
“This is a dangerous game and has consequences,” he added.
Gibraltar has insisted its decision to detain the Iranian tanker was taken alone and not on orders from any government, despite a senior Spanish official previously saying the interception was carried out at the request of the United States.
“All relevant decisions in respect of this matter were taken only as a direct result of the government of Gibraltar having reasonable grounds to believe the vessel was acting in breach of established E.U. sanctions against Syria,” Fabian Picardo, the territory’s chief minister, told reporters. “There has been no political request at any time from any government that Gibraltar should act or not act on one basis or another.”
The detained vessel contained 2.1 million barrels of light crude oil, he added.
The British navy said Thursday it had stopped three Iranian paramilitary vessels from disrupting the passage of a British oil tanker through the Strait of Hormuz, the narrow mouth of the Persian Gulf. The brief but tense standoff stemmed from the U.K.'s role in seizing the Iranian tanker.
In this image from file video provided by UK Ministry of Defence, British navy vessel HMS Montrose escorts another ship during a mission to remove chemical weapons from Syria at sea off the coast of Cyprus in February 2014. Five Iranian Islamic Revolutionary Guard Corps gunboats tried to seize a British oil tanker in the Strait of Hormuz Wednesday but backed off after a British warship approached, a senior U.S. defense official told Fox News.
(AP/UK Ministry of Defence)
On Friday, the British Ministry of Defense said it was moving up its timetable for relieving the HMS Montrose, a frigate operating in the Persian Gulf, with the larger HMS Duncan destroyer in the wake of the recent developments.
"This will ensure that the UK alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping lane."
Iran recently began surpassing uranium enrichment limits set in its 2015 nuclear deal with world powers in response to President Donald Trump's decision to pull the U.S. out of the accord a year ago. He also has re-imposed tough sanctions on Tehran's oil exports, exacerbating an economic crisis that has sent its currency plummeting.
The following day, boats believed to belong to Iran's Islamic Revolution Guard Corps (IRGC) approached the British Heritage tanker and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the ship, a Ministry of Defence spokesman said.
Iran denied any attempted seizure.
HMS Duncan, a type 45 Destroyer, will operate alongside HMS Montrose in the Gulf for a short period, before HMS Montrose goes back to Bahrain for routine maintenance.
A government spokeswoman said: "As part of our long-standing presence in the Gulf, HMS Duncan is deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for pre-planned maintenance and crew changeover.
"This will ensure that the UK, alongside international partners, can continue to support freedom of navigation for vessels transiting through this vital shipping lane."
British ministers and officials have been stressing they do not want tensions with Iran to "escalate".
The risk of sending a second warship to the region is that it'll send the wrong signal. How will Iran view it other than an escalation?
The reality though is there were already plans to send Duncan to join the frigate HMS Montrose already based in the region.
Montrose has been working hard and is due to undergo routine maintenance. Her crew who have been working at a heightened tempo in recent weeks will also need a break.
Both warships will be operating together in the region for a short period. But in the current climate ministers clearly felt it was too risky to allow a gap in providing a military escort to British merchant shipping in the region.
Iran has suggested the UK seized the tanker "at the behest of" the US government "in line with America's hostile policies".
Foreign ministry spokesman Abbas Mousavi said: "This is a dangerous game and we advise them not get involved in this game under America's influence."
He added: "We ask them again to release the tanker immediately, which will be in all countries' interest."
Iran previously claimed the tanker was not bound for Syria and threatened to seize a British oil tanker in retaliation, if the tanker was not released.
Senior Iran cleric Kazem Sedighi was reported by Iranian state TV as saying: "The British will soon get a slap from the strong arms of our establishment for their daring, reckless and negligent act that will make them regret it for good."
Media playback is unsupported on your device
Tensions escalated between the UK and Iran after Britain said the Iranian regime was "almost certainly" responsible for attacks on two oil tankers in June.
The ongoing imprisonment of Nazanin Zaghari-Ratcliffe is also a long-standing bone of contention between the two countries.
The UK continues to press Iran to release the British-Iranian mother who was jailed for five years in 2016 after being convicted for spying, which she denies.
Relations between Iran and the US are also under duress, after the Trump administration pulled out of an international agreement on Tehran's nuclear programme and reinforced punishing sanctions against Iran.
The US blames Iran for attacks on six oil tankers in May and June.
The US said it was talking to a number of countries with the "political will" to support the plans, which would include providing boats to escort commercial ships through the area.
Providing armed naval escorts for commercial shipping is expensive, time-consuming and requires a fair degree of logistical planning and coordination between different countries.
It's been done before in the Gulf, in the late 1980s, when the US Navy escorted Kuwaiti oil tankers during the Iran-Iraq war.
More recently, several navies have provided escorts for shipping passing between Yemen and Somalia, protecting them from attacks by Somali pirates.
But, ultimately, these military escorts can only be a temporary remedy for a much deeper problem that needs resolving.
If tightening sanctions on Iran prevent that country from exporting most of its oil then the temptation by its senior commanders to lash out in response will only grow stronger.
Foreign Secretary Jeremy Hunt has said events in the Gulf showed the Royal Navy needs more warships.
Writing in the Telegraph, the Tory leadership hopeful promised to reverse cuts to the size of the Royal Navy if he becomes prime minister.
His leadership rival, Boris Johnson, has also committed to increasing the UK's defence budget, but BBC defence correspondent Jonathan Beale said his plans appeared more modest.
The Department for Transport said it regularly provided security advice to UK ships in high-risk areas.
The threat level means British ships are advised not to enter Iranian waters, BBC defence correspondent Jonathan Beale said.
Boats believed to belong to Iran's Islamic Revolution Guard Corps (IRGC) approached the British Heritage tanker and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the ship, a Ministry of Defence spokesman said.
He described the Iranians' actions as "contrary to international law".
Guns on HMS Montrose were trained on the Iranian boats as they were ordered to back off, US media reported. The boats heeded the warning and no shots were fired.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian tanker because of evidence it was carrying oil to Syria in breach of EU sanctions.
A spokesman for the Royal Gibraltar Police said they had arrested the captain and chief officer of the Iranian tanker on Thursday, on suspicion of breaching EU sanctions, but neither had been charged.
The BBC has been told British Heritage was near the island of Abu Musa when it was approached by the Iranian boats.
Although Abu Musa is in disputed territorial waters, HMS Montrose remained in international waters throughout.
Defence Secretary Penny Mordaunt said the government was concerned by the incident and urged the Iranian authorities to "de-escalate the situation".
Foreign Secretary Jeremy Hunt added the UK would monitor the situation "very carefully".
Prime Minister Theresa May's official spokesman said the government was "committed to maintaining freedom of navigation in accordance with international law".
A spokesperson for the US State Department condemned Iran's actions and said that Washington would continue to work closely with the UK.
Morgan Ortagus said: "We commend the actions of the Royal Navy in ensuring freedom of navigation and the free flow of commerce through this critically important waterway."
Commander of the US Fifth Fleet Vice Adm Jim Malloy described the incident as "unlawful harassment" and said the fleet would continue to work closely with the Royal Navy to defend "the free flow of commerce".
What does Iran say?
The navy of Iran's Islamic Revolution Guard Corps has denied claims it tried to seize the tanker, Iranian news agencies reported. IRGC's navy said there had been no confrontation with any foreign vessels in the past 24 hours.
Iranian foreign minister Mohammad Javad Zarif said the UK made the claims "for creating tension".
"These claims have no value," Mr Zarif added, according to the Fars news agency.
Tensions grew after the seizure of an Iranian oil tanker by authorities in Gibraltar, assisted by British Royal Marines.
On Thursday an Iranian official told the BBC the seizure was "unnecessary and non-constructive escalation by the UK" and called for the tanker, Grace 1, to be released.
On Wednesday, Iranian President Hassan Rouhani called the UK "scared" and "hopeless" for using Royal Navy warships to shadow another British tanker in the Gulf.
"You, Britain, are the initiator of insecurity and you will realise the consequences later," Mr Rouhani said.
Media playback is unsupported on your device
The Royal Navy has a frigate, four minehunters and a Royal Fleet Auxiliary support ship already stationed in a permanent Naval Support Facility in the region, at Mina Salman in Bahrain.
This is enough to provide reassurance, but probably not to deal with a crisis, BBC defence correspondent Jonathan Beale said.
Ministers would now have to consider sending another Royal Navy ship to the region - but this could further escalate tensions with Iran, our correspondent said.
Foreign Office officials said they were keeping the UK's military posture in the region under constant review but insisted they did not want to see tensions escalate.
Analysis
By Paul Adams, diplomatic correspondent
The government says it absolutely doesn't want to get sucked into a confrontation with Iran, but cannot ignore the fact that life for British-flagged ships in the Gulf is becoming increasingly precarious. Hence the decision to raise the ship security level to its highest category - essentially a warning to British ships not to enter Iranian waters.
This is an unusual but not unprecedented step, a reflection of the government's mounting concern.
With limited military resources in the Gulf (one frigate and four mine countermeasures vessels), there's a limit to how much protection the Royal Navy can offer to British shipping. On any given day, there are between 15 and 30 large UK ships, including oil and gas tankers, in the Gulf, with between one and three transiting the Strait of Hormuz.
The United States is looking to put together a coalition to ensure freedom of navigation through the Gulf, but has yet to share details or make requests of possible partners. With the US and Iran locked in a bitter dispute over the future of the 2015 Iran nuclear deal, some countries who already participate in a 33-nation Combined Maritime Force, based in Bahrain, may be reluctant to get more involved.
Can ships in the area avoid Iranian waters?
The Strait of Hormuz, through which all ships must pass to enter the Gulf, is so narrow - just 21 nautical miles (39km) at its narrowest - that Iranian and Omani territorial waters meet in the middle, BBC Security correspondent Frank Gardner says.
So instead of sailing through international waters, ships must pass through Iranian or Omani territory which both extend 12 nautical miles out from their coasts.
Ships do this under something called Rights of Straits Passage - part of a UN convention which gives ships free passage through the world's chokepoints like the Strait of Gibraltar and the Malacca Strait.
In the case of the Strait of Hormuz, shipping is channelled through two lanes heading in opposite directions, each one two nautical miles wide.
This is called the Traffic Separation Scheme.
Both Iran and the US Navy deploy warships to patrol this area and have narrowly avoided confrontation on several occasions.
Once ships pass through the Strait of Hormuz and enter the Gulf they need to be wary of a contested area around the islands of Abu Musa and the Greater and Lesser Tunbs.
These are claimed by both Iran and the UAE but occupied solely by Iranian forces.
What we know about British Heritage
It is understood British Heritage was not carrying cargo at the time of the incident with the Iranian boats.
The vessel is registered at the port of Douglas, in the Isle of Man.
What are US-Iran tensions about?
The US has blamed Iran for attacks on six oil tankers in May and June.
However, the UK and the rest of Europe were more cautious, fearing this would escalate tensions, BBC diplomatic correspondent James Landale said.
It follows the Trump administration's decision to pull out of an international agreement on Tehran's nuclear programme and reinforce punishing sanctions against Iran.
Iran's ambassador to the UN Majid Takht-Ravanchi told the BBC Europeans must do more to compensate Tehran for economic losses inflicted by US sanctions - or Iran would continue to step up its nuclear programme.
The Department for Transport said it regularly provided security advice to UK ships in high-risk areas.
The threat level means British ships are advised not to enter Iranian waters, BBC defence correspondent Jonathan Beale said.
Boats believed to belong to Iran's Islamic Revolution Guard Corps (IRGC) approached the British Heritage tanker and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the ship, a Ministry of Defence spokesman said.
He described the Iranians' actions as "contrary to international law".
Guns on HMS Montrose were trained on the Iranian boats as they were ordered to back off, US media reported. The boats heeded the warning and no shots were fired.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian tanker because of evidence it was carrying oil to Syria in breach of EU sanctions.
A spokesman for the Royal Gibraltar Police said they had arrested the captain and chief officer of the Iranian tanker on Thursday, on suspicion of breaching EU sanctions, but neither had been charged.
The BBC has been told British Heritage was near the island of Abu Musa when it was approached by the Iranian boats.
Although Abu Musa is in disputed territorial waters, HMS Montrose remained in international waters throughout.
Defence Secretary Penny Mordaunt said the government was concerned by the incident and urged the Iranian authorities to "de-escalate the situation".
Foreign Secretary Jeremy Hunt added the UK would monitor the situation "very carefully".
Prime Minister Theresa May's official spokesman said the government was "committed to maintaining freedom of navigation in accordance with international law".
A spokesperson for the US State Department condemned Iran's actions and said that Washington would continue to work closely with the UK.
Morgan Ortagus said: "We commend the actions of the Royal Navy in ensuring freedom of navigation and the free flow of commerce through this critically important waterway."
Commander of the US Fifth Fleet Vice Adm Jim Malloy described the incident as "unlawful harassment" and said the fleet would continue to work closely with the Royal Navy to defend "the free flow of commerce".
What does Iran say?
The navy of Iran's Islamic Revolution Guard Corps has denied claims it tried to seize the tanker, Iranian news agencies reported. IRGC's navy said there had been no confrontation with any foreign vessels in the past 24 hours.
Iranian foreign minister Mohammad Javad Zarif said the UK made the claims "for creating tension".
"These claims have no value," Mr Zarif added, according to the Fars news agency.
Tensions grew after the seizure of an Iranian oil tanker by authorities in Gibraltar, assisted by British Royal Marines.
On Thursday an Iranian official told the BBC the seizure was "unnecessary and non-constructive escalation by the UK" and called for the tanker, Grace 1, to be released.
On Wednesday, Iranian President Hassan Rouhani called the UK "scared" and "hopeless" for using Royal Navy warships to shadow another British tanker in the Gulf.
"You, Britain, are the initiator of insecurity and you will realise the consequences later," Mr Rouhani said.
Media playback is unsupported on your device
The Royal Navy has a frigate, four minehunters and a Royal Fleet Auxiliary support ship already stationed in a permanent Naval Support Facility in the region, at Mina Salman in Bahrain.
This is enough to provide reassurance, but probably not to deal with a crisis, BBC defence correspondent Jonathan Beale said.
Ministers would now have to consider sending another Royal Navy ship to the region - but this could further escalate tensions with Iran, our correspondent said.
Foreign Office officials said they were keeping the UK's military posture in the region under constant review but insisted they did not want to see tensions escalate.
Analysis
By Paul Adams, diplomatic correspondent
The government says it absolutely doesn't want to get sucked into a confrontation with Iran, but cannot ignore the fact that life for British-flagged ships in the Gulf is becoming increasingly precarious. Hence the decision to raise the ship security level to its highest category - essentially a warning to British ships not to enter Iranian waters.
This is an unusual but not unprecedented step, a reflection of the government's mounting concern.
With limited military resources in the Gulf (one frigate and four mine countermeasures vessels), there's a limit to how much protection the Royal Navy can offer to British shipping. On any given day, there are between 15 and 30 large UK ships, including oil and gas tankers, in the Gulf, with between one and three transiting the Strait of Hormuz.
The United States is looking to put together a coalition to ensure freedom of navigation through the Gulf, but has yet to share details or make requests of possible partners. With the US and Iran locked in a bitter dispute over the future of the 2015 Iran nuclear deal, some countries who already participate in a 33-nation Combined Maritime Force, based in Bahrain, may be reluctant to get more involved.
Can ships in the area avoid Iranian waters?
The Strait of Hormuz, through which all ships must pass to enter the Gulf, is so narrow - just 21 nautical miles (39km) at its narrowest - that Iranian and Omani territorial waters meet in the middle, BBC Security correspondent Frank Gardner says.
So instead of sailing through international waters, ships must pass through Iranian or Omani territory which both extend 12 nautical miles out from their coasts.
Ships do this under something called Rights of Straits Passage - part of a UN convention which gives ships free passage through the world's chokepoints like the Strait of Gibraltar and the Malacca Strait.
In the case of the Strait of Hormuz, shipping is channelled through two lanes heading in opposite directions, each one two nautical miles wide.
This is called the Traffic Separation Scheme.
Both Iran and the US Navy deploy warships to patrol this area and have narrowly avoided confrontation on several occasions.
Once ships pass through the Strait of Hormuz and enter the Gulf they need to be wary of a contested area around the islands of Abu Musa and the Greater and Lesser Tunbs.
These are claimed by both Iran and the UAE but occupied solely by Iranian forces.
What we know about British Heritage
It is understood British Heritage was not carrying cargo at the time of the incident with the Iranian boats.
The vessel is registered at the port of Douglas, in the Isle of Man.
What are US-Iran tensions about?
The US has blamed Iran for attacks on six oil tankers in May and June.
However, the UK and the rest of Europe were more cautious, fearing this would escalate tensions, BBC diplomatic correspondent James Landale said.
It follows the Trump administration's decision to pull out of an international agreement on Tehran's nuclear programme and reinforce punishing sanctions against Iran.
Iran's ambassador to the UN Majid Takht-Ravanchi told the BBC Europeans must do more to compensate Tehran for economic losses inflicted by US sanctions - or Iran would continue to step up its nuclear programme.
The Department for Transport said it regularly provided security advice to UK ships in high-risk areas.
The threat level means British ships are advised not to enter Iranian waters, BBC defence correspondent Jonathan Beale said.
Boats believed to belong to Iran's Islamic Revolution Guard Corps (IRGC) approached the British Heritage tanker and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the ship, a Ministry of Defence spokesman said.
He described the Iranians' actions as "contrary to international law".
Guns on HMS Montrose were trained on the Iranian boats as they were ordered to back off, US media reported. The boats heeded the warning and no shots were fired.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian tanker because of evidence it was carrying oil to Syria in breach of EU sanctions.
A spokesman for the Royal Gibraltar Police said they had arrested the captain and chief officer of the Iranian tanker on Thursday, on suspicion of breaching EU sanctions, but neither had been charged.
The BBC has been told British Heritage was near the island of Abu Musa when it was approached by the Iranian boats.
Although Abu Musa is in disputed territorial waters, HMS Montrose remained in international waters throughout.
Defence Secretary Penny Mordaunt said the government was concerned by the incident and urged the Iranian authorities to "de-escalate the situation".
Foreign Secretary Jeremy Hunt added the UK would monitor the situation "very carefully".
Prime Minister Theresa May's official spokesman said the government was "committed to maintaining freedom of navigation in accordance with international law".
A spokesperson for the US State Department condemned Iran's actions and said that Washington would continue to work closely with the UK.
Morgan Ortagus said: "We commend the actions of the Royal Navy in ensuring freedom of navigation and the free flow of commerce through this critically important waterway."
Commander of the US Fifth Fleet Vice Adm Jim Malloy described the incident as "unlawful harassment" and said the fleet would continue to work closely with the Royal Navy to defend "the free flow of commerce".
What does Iran say?
The navy of Iran's Islamic Revolution Guard Corps has denied claims it tried to seize the tanker, Iranian news agencies reported. IRGC's navy said there had been no confrontation with any foreign vessels in the past 24 hours.
Iranian foreign minister Mohammad Javad Zarif said the UK made the claims "for creating tension".
"These claims have no value," Mr Zarif added, according to the Fars news agency.
Tensions grew after the seizure of an Iranian oil tanker by authorities in Gibraltar, assisted by British Royal Marines.
On Thursday an Iranian official told the BBC the seizure was "unnecessary and non-constructive escalation by the UK" and called for the tanker, Grace 1, to be released.
On Wednesday, Iranian President Hassan Rouhani called the UK "scared" and "hopeless" for using Royal Navy warships to shadow another British tanker in the Gulf.
"You, Britain, are the initiator of insecurity and you will realise the consequences later," Mr Rouhani said.
Media playback is unsupported on your device
The Royal Navy has a frigate, four minehunters and a Royal Fleet Auxiliary support ship already stationed in a permanent Naval Support Facility in the region, at Mina Salman in Bahrain.
This is enough to provide reassurance, but probably not to deal with a crisis, BBC defence correspondent Jonathan Beale said.
Ministers would now have to consider sending another Royal Navy ship to the region - but this could further escalate tensions with Iran, our correspondent said.
Foreign Office officials said they were keeping the UK's military posture in the region under constant review but insisted they did not want to see tensions escalate.
Analysis
By Paul Adams, diplomatic correspondent
The government says it absolutely doesn't want to get sucked into a confrontation with Iran, but cannot ignore the fact that life for British-flagged ships in the Gulf is becoming increasingly precarious. Hence the decision to raise the ship security level to its highest category - essentially a warning to British ships not to enter Iranian waters.
This is an unusual but not unprecedented step, a reflection of the government's mounting concern.
With limited military resources in the Gulf (one frigate and four mine countermeasures vessels), there's a limit to how much protection the Royal Navy can offer to British shipping. On any given day, there are between 15 and 30 large UK ships, including oil and gas tankers, in the Gulf, with between one and three transiting the Strait of Hormuz.
The United States is looking to put together a coalition to ensure freedom of navigation through the Gulf, but has yet to share details or make requests of possible partners. With the US and Iran locked in a bitter dispute over the future of the 2015 Iran nuclear deal, some countries who already participate in a 33-nation Combined Maritime Force, based in Bahrain, may be reluctant to get more involved.
Can ships in the area avoid Iranian waters?
The Strait of Hormuz, through which all ships must pass to enter the Gulf, is so narrow - just 21 nautical miles (39km) at its narrowest - that Iranian and Omani territorial waters meet in the middle, BBC Security correspondent Frank Gardner says.
So instead of sailing through international waters, ships must pass through Iranian or Omani territory which both extend 12 nautical miles out from their coasts.
Ships do this under something called Rights of Straits Passage - part of a UN convention which gives ships free passage through the world's chokepoints like the Strait of Gibraltar and the Malacca Strait.
In the case of the Strait of Hormuz, shipping is channelled through two lanes heading in opposite directions, each one two nautical miles wide.
This is called the Traffic Separation Scheme.
Both Iran and the US Navy deploy warships to patrol this area and have narrowly avoided confrontation on several occasions.
Once ships pass through the Strait of Hormuz and enter the Gulf they need to be wary of a contested area around the islands of Abu Musa and the Greater and Lesser Tunbs.
These are claimed by both Iran and the UAE but occupied solely by Iranian forces.
What we know about British Heritage
It is understood British Heritage was not carrying cargo at the time of the incident with the Iranian boats.
The vessel is registered at the port of Douglas, in the Isle of Man.
What are US-Iran tensions about?
The US has blamed Iran for attacks on six oil tankers in May and June.
However, the UK and the rest of Europe were more cautious, fearing this would escalate tensions, BBC diplomatic correspondent James Landale said.
It follows the Trump administration's decision to pull out of an international agreement on Tehran's nuclear programme and reinforce punishing sanctions against Iran.
Iran's ambassador to the UN Majid Takht-Ravanchi told the BBC Europeans must do more to compensate Tehran for economic losses inflicted by US sanctions - or Iran would continue to step up its nuclear programme.
The Department for Transport said it regularly provided security advice to UK ships in high-risk areas.
The threat level means British ships are advised not to enter Iranian waters, BBC defence correspondent Jonathan Beale said.
Boats believed to belong to Iran's Islamic Revolution Guard Corps (IRGC) approached the British Heritage tanker and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the ship, a Ministry of Defence spokesman said.
He described the Iranians' actions as "contrary to international law".
Guns on HMS Montrose were trained on the Iranian boats as they were ordered to back off, US media reported. The boats heeded the warning and no shots were fired.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian tanker because of evidence it was carrying oil to Syria in breach of EU sanctions.
A spokesman for the Royal Gibraltar Police said they had arrested the captain and chief officer of the Iranian tanker on Thursday, on suspicion of breaching EU sanctions, but neither had been charged.
The BBC has been told British Heritage was near the island of Abu Musa when it was approached by the Iranian boats.
Although Abu Musa is in disputed territorial waters, HMS Montrose remained in international waters throughout.
Defence Secretary Penny Mordaunt said the government was concerned by the incident and urged the Iranian authorities to "de-escalate the situation".
Foreign Secretary Jeremy Hunt added the UK would monitor the situation "very carefully".
Prime Minister Theresa May's official spokesman said the government was "committed to maintaining freedom of navigation in accordance with international law".
A spokesperson for the US State Department condemned Iran's actions and said that Washington would continue to work closely with the UK.
Morgan Ortagus said: "We commend the actions of the Royal Navy in ensuring freedom of navigation and the free flow of commerce through this critically important waterway."
Commander of the US Fifth Fleet Vice Adm Jim Malloy described the incident as "unlawful harassment" and said the fleet would continue to work closely with the Royal Navy to defend "the free flow of commerce".
What does Iran say?
The navy of Iran's Islamic Revolution Guard Corps has denied claims it tried to seize the tanker, Iranian news agencies reported. IRGC's navy said there had been no confrontation with any foreign vessels in the past 24 hours.
Iranian foreign minister Mohammad Javad Zarif said the UK made the claims "for creating tension".
"These claims have no value," Mr Zarif added, according to the Fars news agency.
Tensions grew after the seizure of an Iranian oil tanker by authorities in Gibraltar, assisted by British Royal Marines.
On Thursday an Iranian official told the BBC the seizure was "unnecessary and non-constructive escalation by the UK" and called for the tanker, Grace 1, to be released.
On Wednesday, Iranian President Hassan Rouhani called the UK "scared" and "hopeless" for using Royal Navy warships to shadow another British tanker in the Gulf.
"You, Britain, are the initiator of insecurity and you will realise the consequences later," Mr Rouhani said.
Media playback is unsupported on your device
The Royal Navy has a frigate, four minehunters and a Royal Fleet Auxiliary support ship already stationed in a permanent Naval Support Facility in the region, at Mina Salman in Bahrain.
This is enough to provide reassurance, but probably not to deal with a crisis, BBC defence correspondent Jonathan Beale said.
Ministers would now have to consider sending another Royal Navy ship to the region - but this could further escalate tensions with Iran, our correspondent said.
Foreign Office officials said they were keeping the UK's military posture in the region under constant review but insisted they did not want to see tensions escalate.
Analysis
By Paul Adams, diplomatic correspondent
The government says it absolutely doesn't want to get sucked into a confrontation with Iran, but cannot ignore the fact that life for British-flagged ships in the Gulf is becoming increasingly precarious. Hence the decision to raise the ship security level to its highest category - essentially a warning to British ships not to enter Iranian waters.
This is an unusual but not unprecedented step, a reflection of the government's mounting concern.
With limited military resources in the Gulf (one frigate and four mine countermeasures vessels), there's a limit to how much protection the Royal Navy can offer to British shipping. On any given day, there are between 15 and 30 large UK ships, including oil and gas tankers, in the Gulf, with between one and three transiting the Strait of Hormuz.
The United States is looking to put together a coalition to ensure freedom of navigation through the Gulf, but has yet to share details or make requests of possible partners. With the US and Iran locked in a bitter dispute over the future of the 2015 Iran nuclear deal, some countries who already participate in a 33-nation Combined Maritime Force, based in Bahrain, may be reluctant to get more involved.
Can ships in the area avoid Iranian waters?
The Strait of Hormuz, through which all ships must pass to enter the Gulf, is so narrow - just 21 nautical miles (39km) at its narrowest - that Iranian and Omani territorial waters meet in the middle, BBC Security correspondent Frank Gardner says.
So instead of sailing through international waters, ships must pass through Iranian or Omani territory which both extend 12 nautical miles out from their coasts.
Ships do this under something called Rights of Straits Passage - part of a UN convention which gives ships free passage through the world's chokepoints like the Strait of Gibraltar and the Malacca Strait.
In the case of the Strait of Hormuz, shipping is channelled through two lanes heading in opposite directions, each one two nautical miles wide.
This is called the Traffic Separation Scheme.
Both Iran and the US Navy deploy warships to patrol this area and have narrowly avoided confrontation on several occasions.
Once ships pass through the Strait of Hormuz and enter the Gulf they need to be wary of a contested area around the islands of Abu Musa and the Greater and Lesser Tunbs.
These are claimed by both Iran and the UAE but occupied solely by Iranian forces.
What we know about British Heritage
It is understood British Heritage was not carrying cargo at the time of the incident with the Iranian boats.
The vessel is registered at the port of Douglas, in the Isle of Man.
What are US-Iran tensions about?
The US has blamed Iran for attacks on six oil tankers in May and June.
However, the UK and the rest of Europe were more cautious, fearing this would escalate tensions, BBC diplomatic correspondent James Landale said.
It follows the Trump administration's decision to pull out of an international agreement on Tehran's nuclear programme and reinforce punishing sanctions against Iran.
Iran's ambassador to the UN Majid Takht-Ravanchi told the BBC Europeans must do more to compensate Tehran for economic losses inflicted by US sanctions - or Iran would continue to step up its nuclear programme.
