A record number of people filed their tax returns online for 2018/19, the government has said.
HMRC said 10.4m self-assessment tax returns were filed online, out of a total of 11.1m received by the deadline of 11:59pm on 31 January.
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But almost 1m people missed the deadline, which can incur an initial fine of £100.
Anyone with more than one source of income and the self-employed must complete their tax returns and send them to HMRC each year.
A breakdown of HMRC’s data also showed 26,562 taxpayers filed with less than 10 minutes to spare before the deadline hit.
Filings peaked between 4 and 4:59pm on Friday with almost 60,000 submissions.
“I’d like to thank everyone who filed and paid on time, but anyone yet to file or pay should contact HMRC straight away because we are here to help,” HMRC’s director general for customer services, Angela MacDonald, said.
Tax returns filed late earn a £100 fine, while HMRC will fine those who file more than three months late £10 per day, up to a maximum charge of £900.
Further charges of £300, or five per cent of the tax liability, can be levied on tax returns not filed after six months.
Those who provide what HMRC deems a “reasonable excuse” can have their fines waived.
Under new guidance introduced last month, businesses with cash flow problems can defer their payments.