Almost 900,000 taxpayers missed the self-assessment filing deadline at midnight on January 31 and face paying fines, according to figures from HM Revenue & Customs (HMRC)
HMRC was expecting 11.13 million tax returns, but only 92% arrived on time – a total of 10.24 million returns.
Late filers will pay around £8.9 million in fines.
At one stage, the HMRC online portal was creaking under the strain as 826 returns were filed each minute between 1pm and 2pm on January 30.
Millions of taxpayers filed their returns during January, said HMRC, with 4.28 million returns or 42% of all returns filed during the month, even though taxpayers could start filing from April 2014.
All late filers will be fined £100 unless they have a reasonable excuse, regardless of whether they owe any tax or are due a refund.
After three months, an extra £10 a day is incurred, up to a maximum of £900.
At three, six and 12 months late, HMRC charges penalties of 5% of any unpaid tax or £300, whichever is greater.
HMRC’s director general of personal tax, Ruth Owen, said: “We broke the record by having an additional 210,000 taxpayers file their returns on time this year
“It’s pleasing to see most people follow the rules and file their returns and pay their tax on time.
“A few missed the deadline, and I would urge them to act sooner rather than later to stop fines and other penalties from stacking up.
Anyone late with filing their return can find advice on the HMRC web site can call the self-assessment helpline on 0300 200 3310.
Meanwhile, HMRC has published a review of fines and penalties imposed for missing online filing deadlines.
HMRC wants to make fines and tax penalties more expensive as more services go digital.
One idea under consideration is a progressive penalty system based on the same principle as how courts issue penalty points for driving offences, so taxpayers who do not follow the rules pay more than those who may miss the one filing deadline.
The tax man wants views from finance professionals, business and individuals about how to structure fines and penalties.
The consultation documents are available online and responses are requested by May 11.