Sir Keir Starmer’s long-awaited Defence Investment Plan has arrived, but the document raises as many fiscal questions as it answers. At its centre sits a £5 billion funding gap within the four-year plan, a requirement for the Ministry of Defence to absorb £10 billion in internal cuts, and a commitment to reach 3% of GDP on defence spending that is explicitly deferred to the next parliament — with no binding date attached. Paul Johnson, the former head of the Institute for Fiscal Studies, has described it as “striking” that the money has simply not been found, a measured verdict that nonetheless carries considerable weight coming from one of Britain’s most respected fiscal analysts.
The structural problem with the plan is not merely one of arithmetic. The commitment to reach 3% of GDP is contingent on decisions that a future government — potentially led by a different prime minister and funded by a different chancellor — would be required to make and finance. The further ambition of reaching 3.5% by 2035 sits even further beyond the current government’s electoral horizon. Chancellor Rachel Reeves has designated defence as the number one priority for the next spending review, language the Treasury deploys sparingly and rarely so far in advance of a review itself, but the spending that designation implies remains entirely unallocated. A political signal, however emphatic, is not a budget line.
Almost a third of the headline defence boost depends on resources to be confirmed at the upcoming Autumn Budget, which means a substantial portion of what is being presented as a defence uplift rests on fiscal decisions not yet taken. This is a meaningful distinction. Governments that announce spending contingent on future budgets are, in effect, making conditional promises, and the conditions here are neither trivial nor guaranteed. The MoD’s requirement to identify £10 billion in efficiency savings compounds the uncertainty, since departmental efficiency programmes of that scale have a mixed record of delivery across successive administrations.
The plan has also produced collateral consequences for domestic infrastructure. Reports indicate that road improvement projects — schemes designed to ease congestion on major routes — face delay or cancellation as funding is redirected toward the defence budget. The trade-off is a legitimate one for a government navigating constrained public finances and a deteriorating European security environment, but it is a trade-off that deserves honest acknowledgement rather than presentation as a cost-free reallocation. Fiscal choices of this kind involve real opportunity costs, and voters and policymakers alike are better served by clarity on that point.
What the plan does establish, at minimum, is a directional commitment: defence spending is rising, the political priority attached to it is higher than at any point in recent years, and the Treasury has at least signalled its intentions for the next review cycle. These are not nothing. The question that remains open — and that the plan conspicuously declines to answer — is how the gap between stated ambition and confirmed funding will be closed, and by whom. Until that question receives a concrete answer, Britain’s defence posture rests partly on aspiration, and aspiration, as any defence planner will confirm, does not fill an order of battle.