Wes Streeting has pledged to follow his predecessor out of the Ministry of Defence if Labour fails to raise military spending.
The newly appointed Defence Secretary, who has been in the post only a matter of days, said he would resign “as a matter of principle” if the commitment is not honoured.
John Healey stepped down from the role after Sir Keir Starmer declined to commit to lifting defence spending to 3 per cent of GDP by 2030.
Pressed on whether missing that pledge would also force him to quit, Mr Streeting replied: “Absolutely.”
His comments came as the new Prime Minister refused to set out a clear timetable for increasing UK defence spending to meet the 2030 target during a Panorama interview due to air tonight.
Mr Streeting said he would not have accepted the defence brief without assurances that Andy Burnham was committed to increasing funding for the armed forces. He added that it would be for Mr Healey, now Chancellor, to draw up the budgets, but said he was “in no doubt” that defence investment would be delivered.
Asked again whether a failure to raise military funding would be a resignation matter, Mr Streeting told Times Radio: “It is absolutely, for me, a matter of principle.”
A source later claimed that Mr Streeting was referring to the broader principle of defence funding, rather than tying his position to a precise spending figure or deadline.

Wes Streeting vowed yesterday to follow his predecessor and quit as Defence Secretary if Labour does not increase military spending

John Healeye said Sir Keir had been ‘unable’ and the Treasury ‘unwilling’ to ‘commit the resources that the nation needs to defend the country at this time of rising threats’
Mr Streeting also accused former chancellor Rachel Reeves and Sir Keir of putting the country’s security at risk by not setting out how to reach the defence spend goal.
Last week the boss of BAE Systems, Europe’s biggest defence contractor, warned that Britain faces ‘by far the most threatening time I’ve seen’.
Mr Streeting resigned as health secretary in May, triggering the downfall of Sir Keir’s government.
Mr Burnham was asked about defence spending by the BBC yesterday and said he was ‘very aware’ of the reasons for Mr Healey’s resignation.
‘The first challenge facing us both is to ensure that the Defence Investment Plan is fully funded. That’s the thing that’s right in front of us and we need to work that through as we go towards the Budget later this year,’ he said.
Last week, Mr Burnham said that he and Mr Healey wanted to ‘honour’ the UK’s Nato commitments. These were made under duress after Donald Trump demanded that European countries ramp up defence spending to 5 per cent of GDP by 2035.
Yesterday, Mr Streeting said Europe should thank the US President for shaking Europe ‘out of our complacency’ over its reliance on the US for defence.
But asked on Sky News’ Sunday Morning with Trevor Phillips if he felt he could trust Mr Trump, he said: ‘I don’t know him, but I do trust the United States.’

Mr Streeting also accused former chancellor Rachel Reeves and Sir Keir of putting the country’s security at risk by not setting out how to reach the defence spend goal

Mr Healey suggested he would find billions more for defence since being appointed to Chancellor
His reticence echoes that of Mr Burnham, who also said yesterday that he would put the UK’s interests first – even if that meant disagreeing with the President.
In his first major interview, with the BBC’s Laura Kuenssberg, Mr Burnham said: ‘I can’t at any point say that I won’t take a different opinion from him [Mr Trump], that I will need to voice a different idea that is right for Britain.’
He said he would publicly disagree with Mr Trump if necessary, adding: ‘You have to defend your own national interest before anything else. That’s what you’re required to do if you’re to do this job properly.’
The leaders spoke over the phone last week, within an hour of Mr Burnham arriving in Downing Street. The 20-minute conversation was described by No 10 as ‘extremely warm’.
Meanwhile, Mr Burnham is expected to visit a naval base with Volodymyr Zelensky today and will announce that he will share technology for an electronic warfare system with Ukraine. The Ukrainian president will be the first foreign leader to visit Downing Street since Mr Burnham took office. The Prime Minister said: ‘Britain stands with Ukraine… Russia should be in no doubt of our resolve and we will not back down until we achieve long-lasting and just peace for Ukraine.’
Mr Streeting is seeking to improve the Government’s relationship with the US, which became strained under Sir Keir over the Iran conflict, by working to reopen the Strait of Hormuz.
He said he had told his US counterpart Pete Hegseth that the UK would not support offensive action in Iran but would work to ensure it didn’t develop nuclear weapons.
It came as Mr Healey suggested he would find billions more for defence since being appointed to the job. The Chancellor told the Sun on Sunday that it was the ‘first duty of any government’ to keep its citizens safe. Sir Keir announced a £15billion increase in UK defence spending last month, but Treasury documents revealed that £4.7billion of the funding has not been identified and will have to be found at the next Budget, and even then it will only lift Britain’s defence budget to 2.7 per cent of GDP by 2029. Defence chiefs say they need £28billion.
In his resignation letter last month, Mr Healey said the defence settlement ‘falls well short of what is required for defence and the country at this dangerous time’.
He said Sir Keir had been ‘unable’ and the Treasury ‘unwilling’ to ‘commit the resources that the nation needs to defend the country at this time of rising threats’.
Armed Forces minister Al Carns also quit amid concerns No 10 would ‘not listen’ over its failure to support the military.