LONDON — Britain’s next prime minister is expected to inherit a multibillion-pound challenge after the government acknowledged that a significant portion of its newly announced defence spending plan remains unfunded, forcing difficult choices over taxation, public spending and fiscal priorities later this year.
The funding gap emerged only hours after Prime Minister Keir Starmer unveiled a long-awaited defence strategy designed to prepare Britain’s armed forces for a rapidly changing security environment marked by Russia’s continued military aggression, growing geopolitical instability and NATO’s expanding defence commitments.
Although the government pledged an additional £15 billion ($6.2 billion) to strengthen Britain’s military capabilities, accompanying budget documents revealed that roughly one-third of the planned increase still lacks an identified source of funding.
The shortfall, estimated at £4.7 billion, means the incoming government—widely expected to be led by Andy Burnham later this month—will have to decide whether to finance the commitment through deeper spending reductions elsewhere in government, higher taxation or increased public borrowing.
The issue has quickly become one of the most significant fiscal challenges awaiting Britain’s next administration.
Defence Minister for Procurement Luke Pollard sought to downplay concerns, arguing that governments frequently announce long-term spending commitments before finalising detailed budget allocations.
“It’s not unusual for governments to make announcements saying this is what we’ll spend, and then to complete the details of that at the next budget,” Pollard told Sky News on Wednesday.
He also expressed confidence that Burnham, if he succeeds Starmer, would maintain Britain’s commitment to strengthening national security.
“I know that if Andy Burnham becomes the prime minister … that he will take national security as seriously as Keir has taken it,” Pollard said.
Burnham has not publicly outlined how he would finance the remaining defence commitments or whether he intends to modify Starmer’s strategy after taking office.
The defence plan was introduced against an increasingly tense international backdrop.
British intelligence assessments, echoed by several NATO allies, have warned that Russia could potentially threaten another NATO member before the end of the decade if European military capabilities are not significantly strengthened.
Those concerns have accelerated efforts across Europe to increase defence budgets following years of underinvestment after the end of the Cold War.
Britain’s strategy aims to rebuild military readiness by expanding investment in equipment, ammunition stockpiles, industrial production capacity and personnel while modernising key defence capabilities.
The government argues that the spending increase is necessary not only to deter future aggression but also to ensure Britain continues meeting its responsibilities within NATO.
However, critics say announcing ambitious defence spending without fully identifying the necessary funding risks undermining confidence in the government’s broader fiscal management.
The timing also places additional pressure on the incoming administration, which will already inherit slowing economic growth, persistent inflationary pressures and continued demands for increased investment across public services.
The defence funding issue therefore extends beyond military policy into wider questions about Britain’s economic priorities.
One of the most contentious aspects of the strategy involves the government’s intention to finance part of the military expansion by reallocating money originally intended for infrastructure and development projects.
Several road, transport and energy initiatives are expected to receive reduced funding under the proposed reallocation.
That decision has prompted criticism from political opponents and economic analysts who argue that infrastructure investment remains essential for long-term productivity growth.
The reductions are particularly sensitive because Starmer’s government was elected in 2024 after promising sustained investment in Britain’s transport networks, renewable energy sector and regional economic development.
Opposition parties have accused the government of abandoning those commitments in favour of defence spending that has not yet been fully financed.
Military experts have also questioned whether the strategy goes far enough.
While broadly welcoming the additional investment, several former senior officers criticised the government for failing to provide a detailed timetable showing when Britain’s defence spending will reach 3% of gross domestic product.
That figure is widely regarded as an intermediate milestone before meeting NATO’s recently agreed objective of spending 3.5% of GDP on defence by 2035.
The alliance adopted the higher target as member states responded to growing security concerns following Russia’s invasion of Ukraine and the broader deterioration in European security.
Britain has publicly supported that objective but has yet to publish a detailed financial pathway explaining how future governments intend to reach it.
The absence of such a roadmap has become another point of criticism.
Economic analysts say the immediate funding gap represents only the beginning of a much larger fiscal challenge.
The Institute for Fiscal Studies estimated that implementing the current plan would require approximately £1.2 billion in additional annual spending before accounting for future commitments associated with NATO’s longer-term targets.
According to the think tank, reaching defence expenditure equivalent to 3.5% of GDP by 2035 could eventually require approximately £25 billion in additional spending every year.
That would force whichever government is in power to make difficult trade-offs between defence, healthcare, education, welfare and infrastructure.
“There will be further impacts on other areas of spending, tax or borrowing on top of those set out in today’s announcements – implying one key early decision for the next prime minister,” the Institute for Fiscal Studies said in its assessment.
“Defence spending will likely remain one of the biggest fiscal pressures facing the UK in the medium term.”
The organisation noted that Britain already faces significant demographic pressures as an ageing population increases demand for healthcare and pension spending.
At the same time, sluggish economic growth has limited the government’s ability to generate additional tax revenue without raising tax rates.
Those structural constraints mean financing higher defence budgets may become increasingly difficult over the coming decade.
Political observers say the incoming government will have limited room for manoeuvre.
Large-scale tax increases would likely prove politically unpopular during a period of elevated living costs.
Additional borrowing could conflict with Britain’s existing fiscal rules and potentially increase pressure on government debt servicing costs as interest rates remain higher than in previous years.
Meanwhile, further reductions in departmental spending risk generating resistance from local authorities and public sector organisations already coping with tight budgets.
The defence review nevertheless reflects a broader shift taking place across Europe.
Governments that once prioritised domestic investment over military expenditure are increasingly directing larger portions of national budgets toward defence following changes in the international security landscape.
For Britain, the challenge is particularly acute because of its long-standing role as one of NATO’s leading military contributors.
Successive governments have emphasised Britain’s importance as a nuclear power, permanent member of the United Nations Security Council and major European military force.
Maintaining those capabilities requires sustained investment across conventional forces, cyber defence, intelligence and industrial production.
Officials argue that delaying investment would ultimately increase costs while weakening Britain’s ability to respond to future threats.
Supporters of the strategy also contend that defence spending generates wider economic benefits through advanced manufacturing, technology development and skilled employment.
However, economists caution that those gains must still be balanced against competing demands elsewhere in the public finances.
As Britain prepares for a change in political leadership, the defence funding gap is likely to become one of the first major tests facing the incoming administration.
Whether the government chooses higher taxes, spending cuts or additional borrowing, each option carries political and economic consequences.
With security concerns continuing to dominate the European agenda, few expect defence spending itself to decline.
Instead, the debate is increasingly shifting toward how Britain intends to pay for a larger military while maintaining investment in other priorities central to economic growth and public services.
The decisions made during the next budget are therefore expected to shape not only Britain’s future defence posture but also the broader direction of government spending for years to come.
