Around three million taxpayers have until midnight on Friday to submit tax returns for 2018/19, according to HMRC.
The Revenue said that more than 8.5m taxpayers have submitted returns through self-assessment one week before the 31 January deadline.
A record-high of 11.7m people are required to complete a self-assessment tax return for any untaxed income earned in 2018/19.
This usually applies to the self-employed and high earners, while income tax and national insurance contributions are paid at the same time.
Anyone who earns more than £1,000 from self-employment or £2,500 from other untaxed income needs to file a tax return.
Penalties of £100 kick in instantly for those who miss the deadline, with additional fines for non-compliance in place thereafter.
Jesse Norman, financial secretary at the Treasury, said:
"The deadline is fast approaching for more than three million taxpayers who have not yet managed to file their tax returns.
"With less than one week until 31 January, HMRC strongly encourages people to file their self-assessment."
Angela MacDonald, director-general of customer services at HMRC, added:
"We're here to help customers get their tax right, and we know completing a tax return can be confusing at times, so please do get in touch if you're still concerned about how to file, or about paying what you owe."