Some 9.24 million people completed their self-assessment tax forms via the HM Revenue and Customs (HMRC) website.
A total of 1.14 million returns, the other 11% of submissions, were filed on paper before an earlier deadline.
Some 870,000 expected returns were not received by the deadline, with those individuals facing financial penalties.
These fines start at Â£100, even if there is no tax to pay, before rising to an additional Â£10 a day after three months, and rising further if returns are more than six and 12 months late.
The busiest period for self-assessment submissions was between 14:00 GMT and 15:00 GMT on Friday, when 50,358 people sent through their forms online.
"Each year, we are dedicated to making the self-assessment process easier and more intuitive," said Ruth Owen, director general for personal tax at HMRC.
The Friday surge came despite many people facing disruption when trying to access HSBC bank accounts online, with the bank being "attacked" by cyber criminals.
A record 89% of self-assessment taxpayers who filed their return by Sunday's deadline did so online, as many turn their back on paper filing.