Defence Secretary John Healey has resigned, he has revealed on social media.
In a post shared early this afternoon, Mr Healey confirmed he has stepped down with "great regret and reluctance".
Healey has accused Prime Minster Sir Keir Starmer of failing to properly fund the Defence Investment Plan (Dip).
The Dip, originally called for by the Strategic Defence Review almost exactly a year ago, has been long delayed by wrangling over funding.
Defence Secretary John Healey confirms resignation
His letter to the Prime Minister opens with: "This is a letter I never expected to write, and I do so now with great regret and reluctance."
It later continues: "This era for defence required further investment through the defence investment plan.
"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 adds: "However, your DIP financial settlement – which I was first given in full on Monday afternoon this week – falls well short of what is required for defence and the country at this dangerous time.
"The extra support is backloaded when the pressure of operations and imperative to speed up readiness to fight is in the first two years and it rises to just 2.68% of GDP in 2030, when we will reach 2.6% next year with the investment we are already making."
The letter concludes by saying: "You know what defence needs. You made the argument for this powerfully in your speech at the Munich Security Conference back in February.
"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.
"After explaining to you that I would not be able to accept a DIP settlement that does not give our forces the resources they need, I am now left with no other option than to submit my resignation as your defence secretary.
"I wish you all continuing strength in the exceptional challenges you face as Prime Minister.
"As always, our Labour Government will continue to have my fullest support."
Healey has served as Secretary of State for Defence since July 2024.
He has been the MP for Rawmarsh and Conisbrough, formerly Wentworth and Dearne, since 1997.
He previously held various junior ministerial positions under Tony Blair and Gordon Brown from 2001 to 2010.
