It emerged shortly after that there was a £4.7 black hole with a decision on where that would come from to be made at the Budget in the autumn. But yesterday, No 10 was unable to give detail on where the remaining £10.3 billion cuts to Whitehall spending which will fund the plan will come from.

Tory Shadow Defence Secretary James Cartlidge said the defence investment plan had “completely unravelled barely a day after it was published”.

He said: “The increase in spending is far too little, the capability promised comes far too late, and now we learn that they don’t even know how to fund it.

“Keir Starmer is kicking the can down the road for his successor because he has failed to take the tough decisions needed to keep the country safe.”

Departments across Government must slash 1% off their spending on major projects to pay for the defence boost, while transport and energy spending will face larger axes.

No 10 revealed that road building projects in Derby and Lincolnshire are among those being sized up in the cuts.

But the Prime Minister’s official spokesman was unable to provide a full list of the projects which will be scrapped to account for the full £10.3bn in savings.

Details will be provided “by the autumn”, he said, also indicating that some hospital building programmes could be cut.

It will prove to be a headache for Mr Burnham, who is set to enter 10 Downing Street within weeks.