The arithmetic is stark. Reeves set £23.6bn of headroom against her primary fiscal rule, which requires a current budget surplus by 2029-30, according to the OBR's October 2024 Economic and Fiscal Outlook. The EY Item Club now estimates that roughly £10bn of that buffer has already been consumed by higher gilt yields and energy-linked spending pressures, as first reported by City AM. The remaining margin may not survive the spending commitments still to come.

For operators running UK businesses, the implications are direct: a heightened probability of further tax rises or spending cuts later in 2025, an energy cost shock already feeding through to input prices, and a borrowing environment that could deteriorate further if gilt markets reprice fiscal risk.

How the OBR's own numbers frame the borrowing risk

In 2024, the OBR modelled what would happen to public finances if the UK faced an energy supply disruption "comparable to the 1973 oil embargo," according to the scenario analysis published alongside the October Budget. The result: an average of £23.1bn per year in additional government borrowing.

The current crisis appears to exceed that scenario's assumptions. The International Energy Agency's executive director, Fatih Birol, said last month that the Hormuz blockade was more serious than the supply shocks of "1973, 1979 and 2022 together," as reported by City AM. Oil prices have risen by 55% and wholesale gas prices by 40% since January, when the OBR last collected market data.

The OBR's modelling assumed oil and wholesale gas prices would remain 75% higher over the course of a year. Current price moves have not yet reached that threshold, but the direction of travel is clear, and the crisis shows no sign of resolution.

Critically, the OBR's scenario was designed as a stress test, not a forecast. It was never scored against the fiscal rules. But the numbers now sit uncomfortably close to reality, and the gap between modelled risk and actual fiscal pressure is narrowing fast.

What £10bn of lost headroom means for tax and spending policy

Matt Swannell, chief economist at the EY Item Club, told City AM that some £10bn would be knocked off Reeves's headroom against the current budget surplus rule. This figure reflects higher interest payments already baked in by movements across gilt yields, before any new spending commitments are scored.

Swannell said that while the scenario analysis would mean the Chancellor still technically meets her fiscal rules, the "narrowing wiggle room" left in public finances would bring discussions around the sustainability of fiscal measures "back into the limelight."

Starting from £23.6bn and subtracting £10bn leaves roughly £13bn of headroom. That sounds like a buffer. It is not. Defence spending pledges and an energy support package, neither of which is yet reflected in OBR forecasts, could absorb most or all of the remainder.

The fiscal framework does contain an escape clause. It allows the government to deviate from borrowing targets in the event of a "significant negative shock" to the economy. Invoking that clause would buy time but could unsettle gilt markets, raising the cost of government borrowing and, by extension, the cost of business borrowing linked to swap rates.

Higher tax receipts from windfall gains on oil and gas profits, plus additional income from interest payments on government-funded loans and investments, could partially offset the pressure, according to City AM's analysis. But few City economists expect these gains to close the gap entirely.

Defence and energy support: the spending commitments still to come

Two large spending demands sit outside current OBR forecasts.

First, the UK has committed under a NATO agreement to reaching 3.5% of GDP on defence and intelligence by 2035. The Defence Investment Plan, which should lay out the costing trajectory, has not yet been published in full. The path from the current spending level to 3.5% implies tens of billions of pounds in cumulative additional expenditure over the next decade, with the early years of ramp-up likely to fall within the current Parliament.

Second, an energy support package for households and businesses is expected no earlier than autumn 2025, according to City AM. The Treasury has breathing room because the fiscal framework stipulates that "temporary measures" lasting fewer than two years do not require a formal OBR assessment. That means the package could be introduced without immediately scoring against the fiscal rules, but the cash still has to come from somewhere.

Together, these commitments represent a structural increase in spending that the current fiscal framework has not absorbed. If scored, they would further erode headroom. If kept off-book through temporary designations or deferred timelines, they risk creating a credibility gap between official forecasts and the actual trajectory of public spending.

What operators should watch for in the autumn fiscal statement

Several variables will determine whether the fiscal squeeze translates into concrete policy changes affecting UK businesses.

Energy prices. If oil remains above 55% higher than January levels and gas above 40%, the OBR's scenario modelling suggests the borrowing impact will intensify. Businesses with energy-intensive operations or long supply chains should monitor wholesale price trends closely.

Gilt yields. City traders at major firms have warned that bond markets could add to the risk premia on gilts if fiscal policy is loosened, according to City AM. Higher gilt yields feed through to swap rates, which in turn raise the cost of fixed-rate business borrowing. Any perception that fiscal discipline is weakening could accelerate this.

Chancellor continuity. Reports have suggested that Energy Secretary Ed Miliband is being considered as a potential successor to Reeves following local elections, with Conservative leader Kemi Badenoch publicly suggesting a reshuffle. A change of Chancellor, particularly one perceived as signalling a leftward shift in fiscal policy, could itself move gilt yields higher.

Tax policy signals. With headroom eroding, the autumn fiscal statement becomes the most likely venue for corrective action. Options include further increases to employer National Insurance contributions, adjustments to capital gains tax rates, or changes to business rates relief. Spending cuts to departmental budgets are the alternative, but politically harder to deliver mid-Parliament.

The OBR's next forecast. The fiscal watchdog's updated projections, expected alongside any autumn statement, will be the definitive assessment of whether Reeves still meets her fiscal rules. If the OBR concludes that headroom has fallen below a credible margin, the political pressure for either tax rises or a formal deviation from the rules will become acute.

The fiscal arithmetic facing UK businesses is not a Westminster abstraction. It is the mechanism through which energy shocks, defence commitments, and borrowing costs translate into the tax and regulatory environment of the next 18 months. The numbers are moving in one direction, and the margin for manoeuvre is shrinking.