UK Debt Passes £3 Trillion as September Interest Costs Reach £10.9 Billion

The Office for National Statistics (ONS) provisionally estimated UK public sector net debt at £3,005.0 billion at the end of September 2026 — just over £3 trillion. It was 94.0% of gross domestic product (GDP), the size of the economy, which was 0.7 percentage points below a year earlier.

September public sector borrowing was £24.5 billion, £1.9 billion above the Office for Budget Responsibility (OBR) forecast. Over the first six months of the UK fiscal year, April through September, borrowing was £97.8 billion: £4.4 billion below the same period a year earlier, but £6 billion above forecast.

Those comparisons answer different questions. Borrowing can improve against last year while still exceeding the amount forecast. Six months of borrowing are not a full-year total, and borrowing during a period is not the same as the stock of debt at its end.

Source: Office for National Statistics (primary statistics) — Public sector finances, UK: September 2026

Central government interest payable in September was £10.9 billion, up 16.6% from a year earlier and £2 billion above forecast. Of that amount, £4.0 billion reflected capital uplift on index-linked gilts — government bonds whose principal adjusts with inflation.

That uplift is recorded as interest payable even though it is not all an immediate cash interest payment in September. Inflation linkage can therefore raise the recorded interest bill without waiting for a bond to be refinanced. For conventional fixed-rate gilts, higher market yields instead reach government costs as new debt is issued and old debt is replaced.

This is a monthly UK central government interest measure, not full-year UK interest or U.S. federal net interest. It should not be compared directly with Treasury net-interest totals without reconciling the coverage and accounting definitions.

Source: Office for National Statistics (primary statistics) — Public sector finances, UK: September 2026

The pound amount measures debt in cash terms. The debt-to-GDP ratio compares that debt with the economy’s size. A cash debt milestone alone cannot show whether the relative burden is rising: the denominator can change too. Here, the reported ratio was below its year-earlier level.

Public sector net debt is also not gross debt. Net debt accounts for specified liquid financial assets under the statistical definition. It is not interchangeable with the gross federal debt shown in LastTrillion’s U.S. headline box. ONS labels these estimates provisional, meaning later information can revise them.

Source: Office for National Statistics (primary statistics) — Public sector finances, UK: September 2026

Analysis: Continuing deficits add borrowing needs. If investors demand higher yields, new and refinanced debt can become more expensive; inflation can separately affect index-linked obligations. Larger interest costs can reduce fiscal space — the room for spending, tax choices or responses to an emergency.

The September release does not prove that deficits alone caused market yields or establish an unavoidable future path. Rates, inflation, growth and policy choices all matter. Reuters provides contemporary reporting on the release; ONS is the primary source for the figures used here.

The useful comparison with France is timing: France’s story is an intraday market signal, while the UK’s is a provisional accounting snapshot with an inflation-linked component. Neither is a U.S. debt measurement, and neither adds a counted Red Flag to the U.S. tracker.

Source: Reuters (contextual reporting) — UK borrows more than expected again as debt hits £3 trillion

Read the separate France bond-yield update →

Read the U.S. interest and refinancing analysis →

On October 27, 2026, the UK Debt Management Office sold £4.5 billion of the 4.625% March 2032 gilt at an average yield of 5.036% — the highest comparable five-year borrowing cost since 2008, according to Reuters. The 4.625% is the gilt’s fixed coupon; the 5.036% average yield is the return investors required at the sale price.

Investors bid 3.51 times the amount on offer, which Reuters described as typical of the strong demand for British government bonds this year. Every gilt was sold. A high auction yield therefore means expensive borrowing, not weak demand: the two are different warnings, and this sale showed the first without the second.

Reuters also reported that 20- and 30-year gilt yields hit their highest since 1998 earlier in October, and that in March the OBR predicted Britain would spend 3.8% of national income servicing public debt by the end of the decade — nearly twice as much as in the 2010s.

Analysis: This is the refinancing mechanism described above operating in real time. As older, cheaper gilts mature and are replaced at yields like 5.036%, interest costs can rise gradually and narrow budget room. The chain is conditional: lower future rates, smaller deficits or stronger growth could ease it. This update changes no U.S. tracker status.

Source: Reuters (market reporting) — Britain issues costliest 5-year debt since 2008 before budget

Source: UK Debt Management Office (primary) — DMO publications and auction results

• Revisions to the provisional September figures and later releases covering more of the fiscal year.
• Whether borrowing keeps exceeding forecast, while retaining the separate year-on-year comparison.
• Inflation-linked uplift, rates on newly issued gilts, and changes in both nominal debt and debt relative to GDP.