Coronavirus cases in the UK have risen by 35,928 - nearly double the number recorded last Sunday, figures show.
Public Health England medical director Yvonne Doyle said the "sharp" rise in cases was of "serious concern".
It comes as Health Secretary Matt Hancock warned that a new variant of the virus was "getting out of control".
Christmas plans have been scrapped or restricted for millions across the UK amid warnings the variant is up to 70% more transmissible than previous types.
The number of new UK infections on Sunday is an all-time high for recorded cases and nearly double the 18,447 cases reported a week ago.
However, it is thought the infection rate was higher during the first peak in the spring, with testing capacity too limited at the time to detect the true number of daily cases.
Prof Doyle said most of the new cases in England were concentrated in London and the South East, although it was too early to say if this was linked to the new variant.
'Awful year'
The government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) estimates the variant could increase the R number by between 0.4 and 0.9, minutes released on Sunday show.
The R number is how many other people one person will infect on average; an epidemic is growing if it rises above 1.
A growing number of countries have banned travel from the UK as a result of this variant, including Ireland, France, Belgium, Italy and the Netherlands.
Eurotunnel is suspending access to its Folkestone terminal from 22:00 GMT for traffic and freight heading to Calais due to the 48-hour travel ban introduced by France.
Speaking on the BBC's Andrew Marr programme, Mr Hancock said the news about the new variant "has been an incredibly difficult end to frankly an awful year".
He said: "Of course we don't want to cancel Christmas... we don't want to take any of these measures, but it's our duty to take them when the evidence is clear."
Susan Hopkins, from Public Health England, told Andrew Marr there was evidence that people with the new strain had "higher viral loads", which meant they were more infectious.
Some 21 million people in England and Wales who entered new restrictions at midnight are being told to stay at home, while non-essential shops and businesses have to close.
Those living under the newly-created tier four restrictions in England will now be unable to mix with other households indoors at Christmas, unless they are part of their existing support bubble.
The health secretary said it was not clear how long the tier four measures would be in place, but it could be for months, "until we can get the vaccine going".
He added that people in tier four should act as if they may have the virus.
In the rest of England, Scotland and Wales, relaxed indoor mixing rules will only apply on Christmas Day.
Covid rules had been relaxed across the UK to allow up to three households to mix indoors for five days over the Christmas period.
A ban on travel between Scotland and the rest of the UK will also apply over the festive period. Police Scotland said it would be doubling its patrols on the borders but it would not be introducing check points.
Mainland Scotland is being placed under the tightest restrictions from Boxing Day.
Wales has also entered a new shutdown, with the health minister saying the new variant was "seeded" in every part of the country.
In Northern Ireland, where the planned relaxation of rules for Christmas is going ahead unchanged, four of the five main parties have called for an urgent meeting to discuss the restrictions.
Northern Ireland is already due to enter a six-week lockdown on Boxing Day.
Readers' stories
People whose Christmas plans were affected as a result of the changes have told the BBC of their anguish at being unable to see loved ones.
Nurse Rachel Adams had been planning to see her parents, who are in their 70s. Her father has prostate cancer.
She lives with her husband and two daughters, aged 18 and 21, in Thame, Oxfordshire, which is in tier two, and her parents live five hours away in Northumberland, which is in tier three.
"I am absolutely heartbroken," she said.
"I am missing potentially the last Christmas with my parents."
Grandmother Gaynor Cawood, who lives near Loughborough in Leicestershire, was expecting to see her son, daughter-in-law and two grandchildren for Christmas.
But she lives in tier three and they live in London, which is now in tier four - meaning a ban on travel to other tiers.
"I can't believe the short notice the government have given us to cancel plans," she says.
"Not only am I now unable to get our Christmas presents to my son, daughter-in-law and two grandchildren - how do you explain to a five-year-old that all the exciting plans we made will now not happen?"
But not everyone will be obeying the restrictions over Christmas.
Alex, a teacher from Huddersfield, which is in tier three, said: "I will be continuing with my plans and meeting family on three days over Christmas."
He said he had recently recovered from Covid-19 and his family are being careful by taking tests and self-isolating.
"As a teacher I'm expected to work till the last day, mixing with 70 random households in an early years bubble, most of which I know do not follow the rules outside of school, or face legal action from [Education Secretary] Gavin Williamson.
"Therefore for three days, when I'm probably safest, as I know we are all OK, I'll continue as normal."
The PM's announcement on Saturday of new restrictions came just days after he defended plans to relax restrictions for five days during the festive period - despite calls by some in the medical profession to scrap the change.
Labour leader Sir Keir Starmer said his party supported the latest restrictions, but he accused Boris Johnson of "gross negligence" in failing to act earlier.
Sir Keir told an online press conference that it was "blatantly obvious last week" that Mr Johnson's plans to relax the rules over Christmas was "a risk too far", adding that his claim that "this is all down to a new form of the virus that has just emerged does not stand up to scrutiny".
London Mayor Sadiq Khan told BBC Breakfast the "11th-hour announcement is a bitter blow" for families and businesses, saying it is the "chop-change, stop-start, that's led to so much anguish, despair, sadness and disappointment".
Tier four restrictions:
Similar to England's second national lockdown - tier four applies to Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It also applies in London (all 32 boroughs and the City of London) and the east of England (Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire and Essex (excluding Colchester, Uttlesford and Tendring).
- Residents told to stay at home, with exemptions for those who have to travel for work or education
- Household mixing indoors is not allowed, unless you live with them, or they are part of your existing support bubble
- All non-essential retail to close, including hairdressers, nail bars, indoor gyms and leisure facilities
- Social mixing cut to meeting one person in an open public space
- Communal religious worship is still allowed
The measures will be reviewed on 30 December.
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How will these latest restrictions affect your plans for Christmas? Email haveyoursay@bbc.co.uk.
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https://news.google.com/__i/rss/rd/articles/CBMiJmh0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL3VrLTU1Mzg4OTE20gEqaHR0cHM6Ly93d3cuYmJjLmNvLnVrL25ld3MvYW1wL3VrLTU1Mzg4OTE2?oc=5
2020-12-20 20:39:00Z
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