Sir Keir Starmer has been rocked by a third resignation after Labour leadership "dark horse" Al Carns quit as Armed Forces Minister.
Just hours after Defence Secretary John Healey tendered his resignation, Mr Carns accused the Prime Minister of "asking our Armed Forces to operate in a more dangerous world on a budget written for a calmer one".
Earlier in the evening, Mr Healey's Parliamentary Private Secretary also quit her frontbench role in another damning resignation over the state of Britain's defence spending.
Pamela Nash, who was elected as the Labour MP for Motherwell, Wishaw & Carluke, told Sir Keir that Labour "must do better".
However, Mr Carns used his 828-word letter to warn the Government's Defence Investment Plan "is not built for the threat we face".
He added: "It is neither transformative enough nor sufficiently funded.
"We are asking our Armed Forces to operate in a more dangerous world on a budget written for a calmer one.
"I have sat in the rooms, seen the assessments, and spoken to the commanders who will be asked to do more with less, and I cannot in good conscience stand at the despatch box and defend a level of investment I know to be inadequate to the task.
Al Carns resigned this evening
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"A serious country funds its defence to meet the threat it actually faces, not the threat it wishes it faced."
Amid fervent leadership speculation, the former Armed Forces minister is widely touted as a potential "dark horse" candidate in a rumoured race to oust the Prime Minister from No10.
Before entering politics, Mr Carns served as a Royal Marines Commando and later became a colonel.
The Birmingham Selly Oak MP completed five tours in Afghanistan and has worked for Tory Defence Secretaries in the past, including Sir Michael Fallon and Penny Mordaunt.
Mr Carns also condemned the "serious problems" presented in the Northern Ireland Legacy Bill.
He added: "I have worked to fix the Bill from the inside, but it remains unfit for purpose. It risks failing the very veterans it claims to protect.
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Al Carn published the damning letter in full online
|X/@ALISTAIRCARNS
"Men and women I served with, those I buried friends alongside, people who did their duty under conditions most individuals in Westminster will never have to imagine.
"I set out the changes I believed were necessary, and the lines which I could not in good conscience go beyond. Those lines have not been accepted."
Mr Carns went on to reveal that he "no longer trusts the process", explaining that he has "run out of room to argue this case honourably from inside Government".
"These two failures are the same failure," he said.
"We ask soldiers to fight for this country. In return, we owe them the kit to do the job and the loyalty to stand by them when it's done.
"We are failing on both. The same failure of seriousness runs through how this country treats the people it asks the most of, in uniform and out of it."
The veteran also laid into Labour's dwindling relationship with the working class, echoing criticism from the likes of Andy Burnham and Angela Rayner.
"Too many working people in this country feel insecure even when they are doing everything right," Mr Carns lamented. "They work hard, contribute, pay their taxes, and still feel one setback away from trouble."
He slammed politics appearing "increasingly performative while everyday life gets harder", adding: "Decisions that should take days, take months. Departments fight each other instead of the problem."