The incoming prime minister will face a substantial fiscal challenge as Labour's newly unveiled Defence Investment Plan leaves a £4.7bn funding gap to be addressed at the next budget.

This shortfall was confirmed within the plan itself, highlighting that only £10.3bn of the £15bn committed has been identified, with the remaining amount needing to be found later this year. This task is likely to fall to the next government, potentially led by Andy Burnham.

John Healey, who resigned as defence secretary due to concerns over the plan's funding, had previously claimed Treasury "trickery" inflated the apparent financial commitment. Dan Jarvis, his successor, managed to secure an additional £1.5bn from the Treasury in his short tenure, bringing the total funding increase to £15bn. However, even Jarvis acknowledged that "we need to do more beyond today's announcement."

Analysts suggest the plan's reliance on £10.7bn in defence efficiencies by 2030, with little detail on how these will be achieved, adds to the uncertainty. Furthermore, the plan assumes defence will be a top priority in the next spending review, potentially leading to cuts in other government departments.

Sir Keir Starmer described the plan as a "platform on which I know my successor will build," but faced criticism from within his own party. Nearly every Labour MP who spoke during the announcement indicated that the current funding was insufficient.

Andy Burnham, widely expected to become the next prime minister, has reportedly held "constructive talks" with Healey on defence spending. However, sources close to Burnham admit his specific philosophy on defence remains unclear, with little emphasis on military matters in his recent public statements.

Jarvis confirmed discussions with Burnham regarding the investment plan but indicated that Burnham was not briefed on all security issues and did not sign off on the entire proposal, leaving questions about his awareness of the £4.7bn funding requirement.

The plan's structure and the acknowledged funding gap present a significant early test for the next leader, forcing a decision on defence priorities amidst other potential budgetary pressures.