Labour’s fiscal headroom has shrunk sharply under the weight of global turmoil, leaving Andy Burnham with a far narrower path to govern than the cushion Rachel Reeves left behind. The chancellor’s March 2026 spring statement carved out £23.6bn of “headroom” against the primary fiscal rule — the requirement to balance day-to-day spending with receipts within five years. Since then, however, the Iran war, rising government borrowing costs and Keir Starmer’s defence investment plan have all eaten into that buffer, and the incoming prime minister will inherit an assessment from the Office for Budget Responsibility that will determine just how much room he has left.
The fiscal inheritance
The starting point for Burnham’s premiership is already constrained by his own commitments: he has pledged to stick to Labour’s existing fiscal rules and to remain consistent with the party’s 2024 manifesto. The OBR will be the judge of whether those rules are being met, and it will weigh a complex mix of headwinds and tailwinds. The Treasury is understood to be preparing to tell Burnham within days that the damage from the Iran war has been less severe than initially feared, delivering only a modest hit to the headroom Reeves set aside. Capital Economics, which estimated in May that the war could erase £10bn from the £23.6bn figure, now expects little change in the OBR’s assessment after a fall in global oil prices and a decline in bond yields since the height of the Middle East crisis.
That relative reprieve should not be mistaken for a clean slate. Reeves herself had already doubled her headroom to £22bn in November 2025 to guard against shocks, and the OBR has downgraded its 2026 growth forecast to 1.1%, down from 1.4%. The headroom figure of £23.6bn was based on projections to 2029/30, but the economic climate has shifted significantly since March.
Defence spending and the funding gap
Outgoing prime minister Keir Starmer this week announced an additional £15bn in defence spending over four years as part of a Defence Investment Plan totalling nearly £300bn. The plan aims to lift defence spending to 2.7% of GDP by the end of the decade — a level not seen in 30 years — and to 4.2% under a broader security commitment. It includes £64bn for the renewal of the nuclear deterrent, £8.6bn for sixth-generation fighter jets under the Global Combat Air Programme with Italy and Japan, and significant investment in drones and autonomous systems. The government expects the plan to create nearly 60,000 direct and indirect UK industry jobs by the end of the decade.
However, the funding for this extra £15bn is far from settled. The Treasury says £10.3bn will come from “reallocating budget” across government departments, but many of the decisions on which budgets will be cut have yet to be made — leaving Burnham with a difficult political task. A further £4.7bn must be found in the autumn budget, representing a shortfall of about £1.2bn a year. Defence chiefs had reportedly sought £28bn in additional funding, meaning the announced figure is well short of what the military wanted. Still, the extra spending could be covered without blowing the fiscal rules, provided the OBR judges the overall position is sustainable.
Global headwinds
The Iran war has been the dominant external shock. The conflict disrupted oil and gas supplies, pushing up energy prices: petrol rose by about 10% between late February and late March 2026, and household gas bills are expected to rise later in the year. Inflation climbed to a three-month high of 3.3% in April, though it eased back to 2.8% in May, unchanged from April. The Bank of England, which kept interest rates on hold at 3.75% in June, had projected CPI to be between 3% and 3.5% in the second and third quarters. Rate cuts, previously anticipated, now appear unlikely, and some analysts have raised the possibility of further hikes.
Economic growth has already shown signs of stalling. The UK grew by 0.6% in the first quarter of 2026, the strongest in a year, but the pace is expected to weaken as the effects of the war feed through. The International Monetary Fund in May predicted UK GDP growth of 1% for 2026, up from a previous 0.8% but down from its January forecast of 1.3%. The OECD is more cautious at 0.9%. The International Monetary Fund projects inflation will peak just below 4% at the end of 2026.
The government’s borrowing costs have risen in tandem. Public sector net debt stood at approximately £2,984bn in May, equivalent to 95.1% of GDP, and debt servicing costs remain among the highest in decades. In 2025/26 the government spent around £110bn on debt interest — about 3.6% of GDP, or roughly 10% of all government spending. In May 2026 alone, debt interest costs surged to a record £11.7bn. Government borrowing in the same month was £23.3bn, a 30.4% increase on the previous year and above the OBR’s forecast. The implied interest rate for 10-year borrowing was around 4.9% in mid-June, while 30-year borrowing stood at 5.5%, though UK 10-year gilt yields had eased to about 4.73% by late June — close to two-month lows.
Options for the autumn budget
Bond markets have so far remained quiet, with yields moving little after Burnham’s scene-setting speech on Monday. His commitment to stick to Labour’s fiscal rules has reassured investors for now, but City analysts are watching his pick for chancellor closely. Any perception that discipline is slipping could provoke a negative market reaction, driving up borrowing costs further.
Beyond the defence commitments, Burnham will also face the challenge of funding emergency energy support and any new policies he wishes to pursue. In that context, analysts at the Swiss bank UBS say a key question will be whether the new prime minister may need to look at tax increases at the autumn budget.
