Taxpayers have until the end of Tuesday to submit their self-assessment returns.
The deadline for submitting online returns and paying any tax owed is midnight on January 31.
The most recent figures from HM Revenue and Customs (HMRC) show that fewer than 2.7 million customers still needed to file their self-assessment returns, as of January 26.
At the same point a year earlier, just under 3.4 million people still needed to file their tax returns.
Just over 12 million taxpayers are expected to file returns.
The deadline for paper returns passed on October 31.
People failing to submit their tax return or pay any tax owed after January 31 may face a penalty.
The penalties for late tax returns include an initial £100 fixed penalty, which applies even if there is no tax to pay, or if the tax due is paid on time.
HMRC has said it will treat those with genuine excuses leniently.
People who are unable to pay what they owe in full may be able to set up a payment plan, allowing them to spread the cost into monthly instalments.
It recently emerged that, earlier on in January, people calling HMRC’s self-assessment helpline faced a 27-minute waiting time on average.
HMRC has estimated that 65% of calls to the self-assessment helpline during the two weeks ending January 8 related to matters that customers could have resolved online.
A letter from HMRC chief executive Jim Harra, written in response to correspondence from the Treasury Committee, said the quickest and easiest way for customers to manage their tax affairs and get answers to their queries is online, through either the HMRC mobile app or the web-based personal tax account.
In addition to filing their return and making payments online, customers can also use HMRC’s digital assistant, published guidance, YouTube videos and live webinars, the letter said.
