British Defence Secretary John Healey unexpectedly resigned on Thursday, saying the government is not willing to spend enough on the military at a time of rising threat.

Healey told Prime Minister Keir Starmer that the government's defence investment plan falls "well short of what is required at this dangerous time."

Publication of the plan has been delayed amid reports of disagreement between the defence ministry and the Treasury.

"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," Healey wrote in a resignation letter.

"I am now left with no other option to submit my resignation as your Defence Secretary," he said.

Healey has been UK defence chief since the Labour Party government was elected in July 2024 and is regarded as a capable and serious minister.

Starmer has pledged to boost UK defence spending to 2.5 per cent of gross domestic product by 2027 and 3 per cent by 2034. But many in the military say that is not fast enough.

The resignation is another blow to the embattled prime minister, who is already facing demands from Labour colleagues that he step down.