One million taxpayers face a £100 fine for failing to submit their self-assessment tax returns on time, HM Revenue and Customs (HMRC) said.
The original online filing deadline of 31 January had been extended to 2 February because of a strike by tax office staff.
Those who missed the deadline now face a £100 fixed penalty and daily charges of £10 will accrue after three months.
A record 9.45 million self-assessment tax returns were submitted on time this year, meaning that 90.4 per cent of taxpayers met the deadline.
The proportion of people who filed online this year also rose to a record 80 per cent – up from 78 per cent last year.
The busiest day for online returns was 31 January, when HMRC received nearly 445,000.
David Gauke, Exchequer Secretary to the Treasury, said: "HMRC has always been clear they want returns, not penalties, so it is good news that over 90 per cent were submitted on time."