Some 7.93 million, or 83%, of the on-time self-assessment returns were sent in online, which is also a new high.
The busiest day for doing so was Thursday, ahead of a midnight deadline.
With 10.34 million people in self-assessment, this means that 93% of taxpayers met the return deadlines of October 31 for paper returns and the end of the day on Thursday for filing online.
Anyone who has not filed their 2011/12 tax return to HM Revenue & Customs (HMRC) will have already incurred a £100 late-filing penalty.
HMRC urged late filers to get their returns in as soon as possible to avoid being hit with further charges.
Some 578,000 taxpayers sent in their returns online on deadline day, topping the 445,000 who did so on the same day last year.
The busiest hour on Thursday was between 4pm and 5pm, when 46,000 returns – more than 12 per second – were received by HMRC.