A total of 746,000 people missed the deadline to file their self-assessment tax return, risking a fine of £100.
Some 11.4 million people, primarily those with more than one source of income and the self-employed, were required to complete returns.
The deadline for those filling in paper forms was the end of October. Online returns should have been completed by the end of Wednesday.
The UK tax authority said a record number filed on time.
HM Revenue and Customs (HMRC) said 10.7 million submitted details on time, but 6.5% missed the deadline, compared with 7% last year.
Angela MacDonald, director general for customer services at HMRC, said: "We want the number missing the deadline to be zero, and we will continue to adapt the process to make it easier and simpler for all our customers until every return is in on time and without avoidable errors.
"If you're one of the small number that missed the deadline, please submit your return now to avoid further penalties. We really don't want penalties, we just want tax returns."
The current system means HMRC could demand a penalty of £100 for late filing during the first three months after the deadline.
After three months, additional penalties of £10 per day can be demanded, up to a maximum of £900, followed by further charges six and 12 months after the deadline.
However, the government is working on plans to introduce a points-based system, similar to driving offences, for those who fail to submit their tax returns on time, rather than an automatic fine.
Under the planned changes, they would instead receive points and have to pay fines after a certain threshold was reached.
Points would also be wiped off the record after a certain period of time.
This could begin with VAT in the 2019 tax year, before income tax is added to the system later, but some accountants are concerned that people may mistakenly believe such a change had already come into force.