Around one million taxpayers failed to submit their self-assessment tax return by the 31 January deadline and now risk being fined, according to HM Revenue and Customs. While the figure is significant, it comes against a backdrop of roughly 11.5 million people who did file on time for the 2024–25 tax year.

HMRC said activity surged in the hours before the cut-off, with tens of thousands submitting returns in the final hour on Saturday night. Nearly half a million people filed on the last day alone, making Saturday afternoon the busiest period for online submissions. To manage demand, the tax authority kept helplines and online support services running throughout the weekend.

Anyone who missed the deadline will automatically be charged a £100 penalty, even if they have no tax to pay or have already settled their bill. Further fines can follow if the return remains outstanding, with daily charges applying after three months and larger penalties added after six and 12 months. Late payment of tax can also attract interest and additional charges.

Self-assessment is required for people with income outside standard PAYE arrangements, such as the self-employed or landlords earning more than £1,000 a year. HMRC said it would consider individual circumstances where there is a reasonable excuse for missing the deadline. Tax specialists advise those appealing a penalty to pay it first to avoid extra interest and to contact HMRC early if they need a payment plan.