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Rishi Sunak has promised billions more for defence to counter threats from "an axis of authoritarian states".
The prime minister said UK military spending would rise to 2.5% of national income by 2030, in a move that hardens a previous spending pledge.
He stressed the UK was "not on the brink of war" but claimed the extra money would put the country's defence industry "on a war footing".
Labour is also committed to a 2.5% target when economic conditions allow.
Labour's shadow defence secretary John Healey said the party "wants to see a fully funded plan" to reach this level, but he said the Tories had "shown time and time again that they cannot be trusted on defence".
He said Labour would review resources for the armed forces within a year of taking office, if it wins the general election later this year.
Making the defence announcement during a visit to Poland, Mr Sunak said the UK was facing the most dangerous international environment since the days of the Cold War between the West and former Soviet Union.
He said the extra spending would put the UK's defence industry "on a war footing," although he stressed the UK was "not on the brink of war".
However, he argued the investment was required because authoritarian states such as Russia and China were showing a "new assertiveness" and were increasingly working together.
The prime minister said spending would rise "steadily in each and every year" before 2030. Downing Street said it would mean £75bn more for the military over the next six years.
It echoes a previous commitment made by former PM Boris Johnson, who promised in 2022 to raise spending to 2.5% of GDP by the end of the decade.
Mr Sunak confirmed earlier the UK will provide an additional £500m for Ukraine this year, on top of £2.5bn already allocated.
He added that the UK could continue to provide "at least the current level of military support to Ukraine for every year it is needed".
The prime minister also argued the 2.5% target could set a "new benchmark" for Nato, whose target for members to spend 2% of GDP on defence is now a decade old.
The UK spent 2.07% of GDP on defence last year, according to the data, but says it plans to spend 2.3% this year, including resources given to Ukraine.
In total 11 of the 31 Nato members last year hit the 2% target, according to the transatlantic alliance's estimates.
Poland was the top spender as a share of its economy, allocating 3.9% of GDP - more than twice the amount it had spent in 2022.
The US was in second place, spending 3.5%, although it is by far the biggest spender overall.
The announcement comes after increasing Tory pressure on defence following the Budget in March, with two ministers publicly urging him to increase spending last month.
Downing Street said its 2.5.% commitment was "fully-funded" - but some experts have previously expressed scepticism over long-term targets.
Paul Johnson, director of the Institute for Fiscal Studies think tank, said the previous, vaguer promise to raise spending to 2.5% of GDP was "not worth the paper it's written on unless accompanied by some sense of how it will be afforded".
Last month, MPs on the Public Accounts Committee (PAC) warned the government currently lacked a "credible plan" to fund the MoD.
Last year the National Audit Office (NAO) said the Ministry of Defence was facing a £16.9bn black hole in its finances - despite an injection of £46.3bn over the next 10 years.
Mr Sunak refused to rule out calling a general election in July when questioned earlier by reporters, repeating his previous comment that it would be "in the second half" of the year.
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2024-04-23 14:27:48Z
CBMiL2h0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL3VrLXBvbGl0aWNzLTY4ODgwMTcx0gEzaHR0cHM6Ly93d3cuYmJjLmNvLnVrL25ld3MvdWstcG9saXRpY3MtNjg4ODAxNzEuYW1w
