UK borrows more than expected as impact of Iran war takes toll

The Numbers

• Public sector net borrowing (May 2026): £23.3bn — second-highest May on record

• First two months of FY2026/27 combined: £46.3bn, £8.9bn higher YoY, £7.7bn ahead of OBR forecast

• Debt interest payments (May): £11.7bn — £4.1bn above May 2025

• Government debt-to-GDP: 95.1% — 0.7pp above spring projection

• Tax receipts (May): £85.5bn, up £3.4bn (+4.1%) YoY

• CPI (last month): 2.8% vs. 2.0% BoE target

• Bank Rate: 3.75%, held at the June meeting

The driver is the Iran conflict. Higher fuel costs lifted CPI and pushed up payments on index-linked gilts. The ONS confirmed spending on debt interest, public services, investment, and benefits all rose faster than tax receipts could absorb. Tom Davies, senior statistician at the ONS, stated the data point cleanly: borrowing in the first two months of the financial year was nearly £9bn higher than the same period of 2025.

Political Risk Crosses Bond Market Risk

The figures landed the same day Andy Burnham won the Makerfield byelection. He is now expected to challenge Keir Starmer for the Labour leadership. If he wins, a new chancellor will be appointed. Ed Miliband and Shabana Mahmood are the names circulating.

The defence investment plan is the immediate trigger. It prompted John Healey's resignation as defence secretary last week. Whoever takes over the Treasury will face two questions simultaneously: how to meet Reeves's existing fiscal rules, and whether to fully fund the defence uplift. The OBR has no answer for either scenario.

Martin Beck, chief economist at WPI Strategy, framed the transmission mechanism: "If investors begin to price in larger deficits or stickier inflation, gilt yields could move higher again, feeding directly into mortgage rates and debt interest costs." That is the line operators should monitor. The OBR forecast is already £7.7bn short on the first two months. If political uncertainty widens that gap, the gilt curve steepens, sterling weakens, and the cost of capital for UK-domiciled businesses rises before the next fiscal event.

Operating Response

• Reprice sterling debt. If runway assumes current rates, stress-test at +75bps on the 10-year gilt.

• Watch the next ONS release. A third consecutive overshoot triggers a market reaction; two is noise.

• Track fuel pass-through. The Iran effect runs through diesel and shipping before it reaches CPI.

• Hedge 12-month GBP/USD exposure. Sterling volatility clusters around UK fiscal events.

• Renegotiate index-linked contracts. CPI at 2.8% versus a 2.0% target changes escalator math.

Chancellor Reeves called the conflict a "folly." That is editorial. The spreadsheet version: borrowing is £5.6bn above plan, debt service is £4.1bn higher year over year, and a leadership transition is now inside the forecast horizon. We treat that as a cost-of-capital event. Founders who do the same will price it correctly; those who wait for political clarity will pay the spread.