An estimated one million people missed the deadline to file their tax return and now face penalties, according to HM Revenue and Customs (HMRC).
The UK's tax authority said 27,456 people filed in the final hour before the cut-off for 2024-25 returns at midnight at the end of Saturday.
After HMRC helplines stayed open and webchat services were extended at the weekend, the busiest hour of people filing through online self-assessment was from 17:00 GMT on Saturday.
Some 475,722 people filed on the final day out of a total of about 11.5 million submissions. Those who missed the deadline face an automatic £100 penalty.
"Thank you to the millions of people and agents who filed their self assessment tax return and paid any tax owed by 31 January. Anyone who missed the deadline should file their return as soon as possible, as penalties and late payment interest may be charged," said Myrtle Lloyd, HMRC's chief customer officer.
Millions of people have tax automatically deducted from their wages through PAYE (pay-as-you-earn), but those with more than one source of income may need to file each year through self-assessment.
They included those earning more than £1,000 in the 2024-25 financial year from self-employment or by letting out a property or land.
Some who were not required to file this time include those earning more than £150,000, whose high income was the only reason they needed to file previously, or those who had switched to paying the high income child benefit charge through PAYE.
A similar number of people missed the filing deadline a year earlier. Anyone doing so faces the following penalties:
HMRC, which raises tens of millions of pounds each year from penalties, will consider customers' reasons for missing the deadline. Those with a reasonable excuse may avoid being fined.
Tax analysts advise anyone appealing to still pay the initial penalty.
"Although it involves shelling out cash, it avoids you paying interest on the penalty itself from the date it became due if you lose your appeal," said Charlene Young, senior pensions and savings expert at investment platform AJ Bell.
"If you don't have an excuse to appeal a fine but still owe money, you might still be able to set up a payment plan to get back on track. It's essential you don't put your head in the sand."
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