Kemi Badenoch put the Prime Minister on notice on Monday to keep his word on defence or resign.

The Tory leader gave Sir Keir Starmer ‘one last chance to be a man of his word’ after he repeatedly claimed the defence of the realm was his priority.

Mrs Badenoch launched a scathing attack on the Prime Minister after defence secretary John Healey quit, along with his deputy, Al Carns, over funding pledges for the Armed Forces.

She said the Labour Government posed a ‘major threat to our national security’ – and offered to lend her MPs’ votes to force through cuts to welfare that would fund the essential increase in defence spending.

Mrs Badenoch said she would shepherd 116 Tory MPs behind the Government to overcome the opposition of hard-Left MPs to benefit cuts. But she warned that to get her support, the Prime Minister had to meet three key tests in the still unpublished Defence Investment Plan (DIP).

First, that the Government achieves at least 3 per cent of GDP on defence spending by 2030 – a £28billion increase.

Second, that funding is immediate and not ‘back-loaded’ until after the next general election.

Third, she demanded that the DIP was ‘transformative’ and focused on fighting wars of the future, not wars of the past with outdated kit and technology.

Her demand for £28billion in new cash for defence is almost three times the amount Chancellor Rachel Reeves is offering new Defence Secretary Dan Jarvis.

Mr Healey resigned after condemning a Treasury offering of a real-terms £10billion over four years as failing to ‘give our forces the resources they need’.

It emerged yesterday that Mr Jarvis failed to secure the promise of any extra cash for the DIP when accepting his Cabinet job.

Instead, he has been told he can ‘reprioritise’ how some of the £10billion will be spent.

Mrs Badenoch warned Sir Keir that if he was unable to ‘provide the leadership’ to deliver a DIP that met her three tests ‘he should resign now and make way for a leader who can’.

She has extended her offer of Commons support to Andy Burnham ahead of his potential return to Parliament and coronation as Labour leader. She wrote to Labour’s Makerfield candidate at the weekend, as well as his rivals for the Labour crown.

Junior defence minister Luke Pollard told MPs yesterday that he was still in post only because Mr Healey asked him not to follow his exit from the MoD. He added: ‘I am still standing here because he asked me to stay, and because we need continuity.’

The defence procurement minister told MPs he had worked in ‘lockstep’ with Mr Healey as he fought to secure more funding.

He appeared to lobby for extra cash, telling a Tory MP he welcomed calls to increase defence spending. He also said the DIP would be published before Nato’s summit in Turkey on July 7.