However, an estimated 1.1 million individuals missed the cutoff, potentially leading to £110 million in late filing penalties alone.

Charlene Young, a pensions and savings expert at AJ Bell, noted that rising interest rates, reduced allowances, and frozen tax thresholds have continued to push people into the self-assessment system.

Despite a relaxation of one earnings threshold, a record number of people, 732,498 filed on 31 January itself, with over 31,000 submitting their returns in the final hour.

Ms Young explained: “Anyone who earned over £150,000 for the year should have filed. This earners trigger was one threshold that increased (up from the £100,000 for the previous year), but the starting point for additional rate tax (45%) was also slashed from £150,000 to £125,140 in April 2023, meaning those people who escaped having to automatically do a return would’ve still paid more in tax last year.”

Those who failed to file by the deadline face a £100 late filing penalty, even if they owe no tax. Daily interest will also accrue on any outstanding tax at an annual rate of 7.25%. Unpaid tax from 2023-24 left outstanding by 1 March could incur an additional 5% penalty.

Ms Young says the HMRC estimate of 1.1 million individuals filing late means “a potential windfall for the tax man of £110 million”.

A system outage at Barclays on deadline day may have prevented some taxpayers from making payments. HMRC has stated it is working with Barclays and that those genuinely affected can appeal late payment fines.

Further reductions in tax-free allowances for capital gains and dividends have also resulted in more people having to declare and pay tax for the first time. The capital gains tax allowance was more than halved to £6,000 for 2023-24 and has since been reduced to £3,000. The dividend allowance fell to £1,000 for 2023-24 and is set to drop to £500 for 2024-25.

Taxpayers who have missed the deadline but owe money are urged to contact HMRC as soon as possible to discuss payment options. Those up to date can apply for a budgeting plan or request a reduction in payments on account if their earnings are expected to decrease.