The MoD’s budget is due to rise by 3.6% in real terms by 2029, under departmental spending plans fixed last year.
The increase was weighted towards the long-term investment budget, which pays for new military kit, rather then the day-to-day budget that covers things like administration costs and salaries.
Labour has pledged to raise overall defence spending from 2.3% to 2.5% of national income by 2027, estimated to cost an extra £6bn a year.
It has then promised it will rise to 3.5% by 2035, in line with a pledge made by Nato countries at a summit last year.
This is not the first time that there have been warnings about the MoD’s ability to fund its spending commitments.
The investment plan is meant to replace decade-long “equipment plans” released on a rolling yearly basis until 2022, when the department halted publication whilst it assessed the effect of “extraordinary inflation” on its plans.
An analysis published by the department in December 2023, under the previous government, found that the MoD’s most recent equipment plan was forecast to exceed its budget by £16.9bn.
A report by MPs published the following year said the biggest cause of that shortfall came from spiralling costs from maintaining the UK’s nuclear weapons system, alongside inflation.
The delay to the investment plan has also delayed the release of a separate document detailing yet to be specified “productivity savings” within the department worth £6bn between now and 2029.
An MoD spokesperson said it was working “flat out” to finish the investment plan, adding that Labour had inherited an “underfunded defence programme” from the previous government.