Addressing the PM, Healey wrote: ""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 added.

"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."

He stated that; "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."

"You know what defence needs. You made the argument for this powerfully in your speech at the Munich Security Conference back in February," he wrote.

"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," he continued.

"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," the letter said.

Later on Thursday night, the armed forces minister Alistair Carns tendered his resignation, dealing a double blow to Starmer's government.

In his resignation letter, Carns argued that "it has become clear to me that the change I had pushed for is not going to come."