The UK government borrowed £24.5bn in September, overshooting the Office for Budget Responsibility's forecast by £1.9bn and up sharply from £17bn in August. The culprit is mechanical: inflation-linked government bonds, whose coupon payments rise automatically with the retail prices index. RPI has climbed as the Iran war pushes energy costs higher. The ONS attributes £4bn of the month's debt interest bill directly to that transmission mechanism. Debt interest alone hit £10.9bn — the highest September figure on record, and only the eighth time a single month has breached £10bn. This is not a policy choice; it is the structural consequence of having issued roughly a quarter of UK gilts as index-linked during the low-inflation era. When inflation spikes, the cost of servicing that stock spikes mechanically, regardless of which party holds the Treasury. Total public debt has now crossed £3 trillion for the first time. As a share of GDP, it has actually ticked down — from 95% to 94% — because nominal GDP grows with inflation too. But that statistical comfort is cold: the interest payments are real cash out the door, not ratios on a chart. Every billion spent servicing debt is a billion unavailable for services, investment, or tax relief. For the financial year to date, net borrowing stands at £97.8bn — £4.4bn lower than the same period last year, but £6bn above OBR projections. The gap between forecast and reality is the story that matters for next Wednesday's budget. Chancellor John Healey inherits Rachel Reeves' fiscal rules and her £24bn headroom buffer, but that buffer was calculated before the Iran war repriced the government's own cost of borrowing. Healey is widely expected to accept a thinner headroom margin. This is not fiscal recklessness — it is arithmetic. When your debt stock is inflation-linked and inflation rises exogenously, your headroom shrinks whether you spend or not. The fiscal rules become a straitjacket tightened by events outside the Chancellor's control. Higher inflation does boost tax receipts — VAT, income tax, and corporation tax all rise with nominal activity. But that upside is diffuse and lagged, while the debt interest hit is immediate and concentrated. Government departments operating on fixed nominal budgets see their purchasing power eroded in real time. The net effect is a fiscal squeeze that looks like austerity without anyone having chosen it. The twenty-year question is whether the UK's inflation-linked gilt structure is a design flaw or a feature operating as intended. It was built to give investors confidence and reduce borrowing costs in normal times. In an era of repeated supply shocks — pandemics, wars, energy transitions — it functions as an automatic destabiliser, transmitting every price shock directly into the public finances at the worst possible moment.
