The number of taxpayers who missed the self assessment deadline rose to 958,296 this year, according to data published by HMRC.
However, a record-breaking 11.1 million taxpayers did meet the 31 January deadline, with the number filing online soaring to over 10.4 million for the first time.
Almost 11.7 million customers were required to file their 2018/19 tax returns by the end of January.
More than 700,000 submitted their tax returns on deadline day. The peak hour for filing was between 4pm and 4:59pm, when 56,969 people filed a return, while another 26,562 taxpayers completed their returns in the final hour.
Commenting on the numbers, Angela MacDonald, HMRC's Director General for Customer Services, said: '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.
'Customers who have missed the deadline should contact HMRC. The department will treat those with genuine excuses leniently, as it focuses penalties on those who persistently fail to complete their tax returns and deliberate tax evaders. The excuse must be genuine and HMRC may ask for evidence.'