A rise in self-assessment and capital gains tax receipts gave the UK’s public finances a smaller than expected £15.4bn lift in January.

The surplus is still the highest since records began in 1993 and is a reversal of December’s slump, when the public finances slid to a £17.8bn deficit.

However, in a blow to the chancellor, Rachel Reeves, last month’s figure came in below the predictions of City economists and the government’s independent forecaster, the Office for Budget Responsibility, who had expected a surplus of £20bn.

Reeves needs a more substantial boost to the public finances to stay within the government’s fiscal rules, which put limits on the annual deficit and the level of debt by the end of the parliament.

Reeves is due to make her spring statement to the House of Commons on 26 March, when she is likely to revise some spending plans to stay within the budget constraints.

Demands for extra spending on public services, and the need to increase defence spending to join European efforts to support Ukraine, are being considered inside the Treasury while the bill for borrowing on international financial markets has increased.

A rise in the interest rate on government borrowing last month proved short-lived but the cost of monthly debt payments remains elevated.

Reeves said on Thursday that the UK would need to find funds to support higher military spending, but this would need to be found from savings elsewhere in the government budget plans.

Responding to the January finances figures, the chief secretary to the Treasury, Darren Jones, said they highlighted the need to examine “every pound spent, line by line”, which he said was being done for the first time in 17 years, “ensuring every penny delivers on the country’s priorities in our plan for change”.

There are fears among Labour backbench MPs that many of the initiatives to improve public services with extra investment will be shelved and welfare claimants will have a tougher time.

The Office for National Statistics (ONS) said borrowing in the financial year from April 2024 to January was £118.2bn, up £11.6bn on the same period in the last financial year and the fourth-highest financial year-to-January borrowing since monthly records began in 1993.

Debt interest payments reached £6.5bn in January alone, the second highest January figure since monthly records began 27 years ago.

Nabil Taleb, an economist at the accountants PwC, said: “This reflects the fiscal challenge the chancellor faces. Higher debt servicing costs as a share of total revenues will leave the public finances more exposed to future economic shocks.”

Alex Kerr, an economist at the consultants Capital Economics, said January’s figures “will do nothing to reduce the chancellor’s challenges. Even before the ratcheting up of pressure on European governments to increase defence spending, the chancellor’s options ahead of the fiscal update next month were bleak.”

The ONS said that taken together, self-assessed income and capital gains tax receipts were provisionally estimated at £36.2bn in January 2025, £3.8bn more than a year earlier, and the highest January receipts since monthly records began in 1999.

The chancellor usually enjoys a spending surplus in the first month of the year, which coincides with the deadline for self-assessment returns. The boost usually extends into February, when millions of tax returns that are filed at the last minute are counted by HMRC.

However, last month’s self-assessment haul was £3bn lower than had been forecast, while the CGT take was £1.1bn under the prediction.

Increases in capital gains tax on the sale of business assets, due to take effect in April, are expected to continue bringing forward receipts as company owners seek to beat the deadline.