Dan Jarvis, the new defence secretary, will have to make 'very significant cuts' in his department if he cannot secure more cash in the next two weeks. 

The offer made by Sir Keir Starmer to increase defence spending by £13.5billion over the next four years, shared with Jarvis' predecessor John Healey before he resigned, would have left the Ministry of Defence without enough money for its existing equipment programme. 

It would have meant that the strategic defence review - the UK's plan to make Britain 'secure at home and strong abroad' - unachievable, the Times reports. 

Officials working to balance the books reportedly considered delaying new investments in drones and AI. They were also looking into the cancellation of reserve days and military exercises. 

The Times reported that of the £13.5billion offered to Healey before his resignation on Thursday, £3.5billion would have been made by 'changing contingency levels' with the MoD's budget. 

The defence ministry has spare cash that it can use in crises, when inflation is higher than expected and when it overspends. 

A source added that this £3.5billion would have included the Treasury 'taking on the cost of things the MoD was never going to pay for anyway'.

The offer made to Healey infuriated him, and in his resignation letter he said it fell 'well short of what is required.'

Healey sternly added: 'You know what defence needs. You made the argument for this powerfully in your speech at the Munich Security Conference back in February. 

'Without a [Defence Investment Plan] 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.' 

On Monday, Tory leader Kemi Badenoch gave Starmer 'one last chance to be a man of his word' after he repeatedly claimed the defence of the realm was his priority. 

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 But she warned that to get her support, the Prime Minister had to meet three key tests in the still unpublished Defence Investment Plan (DIP).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. 

The Daily Mail has contacted the Ministry of Defence for comment.