Andy Burnham refused yesterday to set out a timetable for spending 3 per cent of the UK's GDP on defence.
Questioned by Conservative leader Kemi Badenoch, the Prime Minister did not give a date for meeting such a target.
Instead, he claimed that he had appointed John Healey as his Chancellor 'precisely because of his commitment to defence'.
He insisted Mr Healey would unveil the plans to spend 3 per cent and then 3.5 per cent of GDP on defence at next year's spending review.
Sir Keir Starmer's former defence secretary quit after accusing the then-PM of putting the country's security at risk by not fully funding the defence investment plan.
In his resignation letter, he said that he was 'certain that a headmark date for 3 per cent of GDP on defence in 2030 is what Britain must set' ahead of the Nato commitment of spending 3.5 per cent by 2035.
But there have been suggestions that he could backtrack on this after he said that being Chancellor was 'a bigger job' than his previous role.
Mr Healey failed to address those fears as he yet again refused to commit to a timetable.
Prime Minister Andy Burnham refused to commit to a timetable for meeting the target of spending 3 per cent of GDP on defence after his Chancellor John Healey did the same
In her first exchange in the Commons with the new Prime Minister, Mrs Badenoch demanded to know when the Government would meet the 3 per cent goal.
She asked him: 'Will he provide the funding which his own Chancellor said was necessary to keep British troops safe?'
Mr Burnham replied: 'I appointed this Chancellor of the Exchequer precisely because of his commitment to the defence of our realm and the arguments he has made, and he will set out at the spending review the path to honouring our international commitments in full.'
Britain's problems? I blame Thatcher and Brexit, says Burnham as he made his Commons debut as PM
Mr Healey himself had said that he would reveal when the country would get to 3 per cent, though he had refused to say when.
However, he then told one newspaper: 'For the first time at the [spring 2027] spending review, we will set out a clear path about how we will meet that Nato commitment in 2035, of 3.5 per cent of GDP. And as part of setting out that clear path, we will set that target date for 3 per cent.'
And Mr Healey vowed defence spending would be central to his spending plans following fears that he would defer the target.
Yet he warned that filling a £5billion hole in funding would mean 'difficult decisions' at the Budget next month.
He insisted that he remained '100 per cent' committed to delivering on the Nato pledge of spending 3.5 per cent of GDP on defence by 2035.
Earlier, Defence Secretary Wes Streeting insisted: 'There should be no doubt about the Chancellor's commitment to investing in our defence.'
Yet he has also declined to commit to such a target but instead added: 'I certainly feel like we have a firm friend in His Majesty's Treasury.'
Defence Secretary Wes Streeting said Mr Healey was a 'firm friend' of his department despite the Chancellor refusing to commit to the target which prompted his resignation from Sir Keir Starmer's government
Mr Healey's resignation as defence secretary proved to be the final blow that ended the premiership of Sir Keir.
The former prime minister set out plans to increase defence spending to 2.7 per cent of GDP by 2027/28 with no further details.
The Treasury has also now stated the Government will reveal plans for reaching both targets at the spending review.
Committing to 3 per cent by 2030 would cost the UK an additional £17.3billion, according to the Office for Budget Responsibility.
James Cartlidge, shadow defence secretary, has accused Mr Healey of 'backtracking on the very pledge which sealed the fate of the last prime minister and opened the door of No 10 to Andy Burnham'.
Mr Healey has previously stated that 'fiscal discipline' is the 'first priority' for any chancellor, adding that the Government 'simply can't deal with the problems we face as a country by maxing out the country's credit card'.
Mr Burnham has said in recent weeks that the fiscal backdrop to the Budget was 'challenging'.