Defence Secretary John Healey has unexpectedly resigned from the Cabinet.
His departure comes after Sir Keir Starmer left him with "no other option" after disagreement over the Defence Investment Plan.
In a resignation letter, Mr Healey criticised Sir Keir Starmer’s unwillingness to commit enough resources to defence.
He wrote: "This new era for defence required further investment through the Defence Investment Plan."
He added: "The excellent and extensive cross-government work that completed in January - overseen by you, me and the Chancellor - confirmed the scale of the challenge and the rising demands on defence.
“Since then, you have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country at this time of rising threats."
He said that "demands on defence have increased still further”.
The former minister pointed to the Iran war, threats from Russia in the High North, and escalation in the Ukraine conflict.
Mr Healey said the proposal for spending in the Defence Investment Plan that he was given on Monday "falls well short" of what is required as the country faces a "dangerous time".
While the Government has committed to spending 3.5 per cent of GDP on defence by 2035, Mr Healey revealed that the Prime Minister had only offered a 0.08 per cent increase in defence spending by 2030.
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Mr Healey posted a damning letter to the Prime Minister
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The figure would rise from 2.6 per cent to 2.68 per cent under the new plan, according to the former secretary.
The MP also criticised the plan's financial support as heavily "backloaded" toward the end of the decade.
The former minister said he had outlined "credible ways" to meet funding challenges in his resignation letter to the Prime Minister as Defence Secretary.
He added: "Without a DIP that meets the moment in this way, I am being forced to make decisions that would reduce the readiness of our forces and increase the risk to personnel on operations, and could make the country less safe."
The Defence Investment Plan, originally called for by the Strategic Defence Review almost exactly a year ago, has been long delayed by wrangling over funding.
It has dominated political conversation throughout the week, with Kemi Badenoch taking aim at Labour on the plans during Prime Minister's Questions yesterday, accusing Sir Keir of "dithering" and "total paralysis".
Mr Healey said the proposal for spending in the Defence Investment Plan that he was given on Monday 'falls well short' of what is required
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Tory leader has now said that Mr Starmer’s "premiership is falling apart", telling reporters today: "His health secretary resigned two weeks ago. His defence secretary has resigned at a critical time when we are facing global threats, and he is doing so because the Prime Minister is trying to please his backbenchers by putting money into welfare instead of defence.
“We need to start funding defence. We need to get to 3% of GDP by the end of this Parliament… Keir Starmer has no plan whatsoever. I don’t see how he can stay in this job. He can’t run the country. He is paralysed because his backbenchers only want to spend money on welfare.”
Reform UK's Robert Jenrick has praised John Healey over the resignation, writing: "This Government has all the money in the world for Ed Milliband’s mad plans, foreign aid, and benefits for foreigners. But nothing for our armed forces.
"Good on John Healey. Shame on them. Reeves and Starmer should go too. And with them this wretched Labour Government."
Sir Ed Davey said today has been a "wake-up" call for the Labour leadership.
The Liberal Democrat leader added in a statement: "Stop repeating the mistakes of the Conservatives and get serious about funding our armed forces properly. We cannot afford years more political chaos while our national security is put at risk.”
John Healey said the Defence Investment Plan, as proposed by Sir Keir Starmer, 'could make the country less safe'
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A YouGov poll reveals that the majority of voters believe that the Labour Government is handling defence badly.
Mr Starmer’s administration was doing a bad job on military issues, 51 per cent claimed, while just 27 per cent said the Government was handling defence well.
Donald Trump has repeatedly urged European allies to dramatically increase military spending, demanding that Nato countries, including the UK, have greater financial responsibility for their own defence.
Chancellor Rachel Reeves has also warned that "borrowing can't be the only answer" to fund defence, hinting at further tax rises in this year's autumn budget.
Mr Healey marks the second cabinet minister to resign following the local election losses, after former Health Secretary Wes Streeting quit on May 14.
Four other ministers have resigned from Starmer's administration, including Jess Phillips, Zubir Ahmed, Alex Davies-Jones, and Miatta Fahnbulleh.
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