He was followed by veteran and armed forces minister Al Carns, as well as two parliamentary aides. In a letter announcing his decision, Mr Healey suggested the Prime Minister had been “unable” to overrule Chancellor Rachel Reeves when it came to securing the amount of cash military officials had hoped for in the Defence Investment Plan (Dip).

The Dip, a blueprint for how the armed forces will be funded into the future, has been long delayed because of wrangling between the Ministry of Defence, the Treasury, and No 10.

Mr Healey warned the Prime Minister that without a Dip that “meets the moment” he would be “forced to make decisions” that “could make our country less safe”.

Meanwhile, Mr Carns said he would not be able to “defend a level of investment I know to be inadequate to the task” if he remained as a minister.

“A serious country funds its defence to meet the threat it actually faces, not the threat it wishes it faced,” he added in his resignation letter.

Mr Healey and Mr Carns were followed out of the door by parliamentary private secretaries (PPS) Pamela Nash and Rachel Hopkins.

Nearly nine hours after Mr Healey’s departure, Dan Jarvis was announced by Downing Street as his successor as Defence Secretary.

The latest resignations comes after Wes Streeting and a host of junior Government figures quit in the wake of Labour's local election drubbing.

And they come as Sir Keir is facing the threat of a leadership challenge as Andy Burnham eyes a return to Parliament in the Makerfield by-election.