The United Kingdom's new Defence Investment Plan, unveiled by the government, has left a substantial funding gap of £4.7 billion that will likely fall to the next prime minister to address.

This unfunded portion of the £15 billion commitment for the armed forces has emerged as a significant financial challenge, potentially impacting future budgets and departmental spending.

While the plan includes an additional £1.5 billion secured from the Treasury by the current defence secretary, Dan Jarvis, in the weeks since taking office, former defence secretary John Healey had previously resigned over concerns about the plan's inadequate funding. Healey's allies had claimed that the initial figures presented were inflated by Treasury accounting.

Details within the plan confirm that only £10.3 billion of the promised £15 billion has been definitively identified. The remaining £4.7 billion must be found at the next Budget, scheduled for the autumn, a task expected to fall to the incoming administration.

Furthermore, the plan relies on achieving £10.7 billion in defence efficiencies by 2030, with limited explanation on how these savings will be realized. It also presumes that defence will be a high priority in the next spending review, potentially leading to cuts in other government departments.

This situation raises questions about the defence priorities of potential future leaders, such as Andy Burnham, who is understood to have had constructive discussions about defence spending but whose specific philosophy on the matter remains unclear.

Dan Jarvis indicated to BBC Newsnight that he had discussed the investment plan with Burnham, though it is understood Burnham could not be briefed on all security matters and did not approve the complete plan. The defence secretary did not confirm whether Burnham was informed about the necessity of finding £4.7 billion to fund the plan.

Many Labour MPs have publicly stated that the current funding levels are insufficient, highlighting a recurring concern about the adequacy of resources allocated to the armed forces.