An estimated one million taxpayers missed the self assessment deadline for the 2024/25 tax year, according to HMRC.
Over 11.48 million taxpayers filed their self assessment tax returns before midnight on 31 January.
However, more than 12 million self assessment taxpayers were expected to file a tax return and pay any tax owed by the deadline.
HMRC says that anyone who needs to file a return and missed the deadline should meet their tax obligations as soon as possible, as late filing and late payment penalties are charged.
The tax authority said that 97.25% of tax returns were filed online with 475,722 taxpayers waiting until the final day to file their return.
On 31 January, 27,456 people submitted their returns in the final hour while the busiest hour for submitting a return was 17:00 to 17:59, when 32,982 people filed.
HMRC advisers handled 5,409 webchats and 10,483 calls to the helplines which, unusually, were opened on a Saturday to provide extra support to taxpayers on deadline day.
Myrtle Lloyd, HMRC's Chief Customer Officer, said:
'Thank you to the millions of people and agents who filed their self assessment tax return and paid any tax owed by 31 January.
'HMRC digital channels are always the quickest and easiest way for people to sort their tax affairs.'
Internet link: HMRC