Self-assessment taxpayers have until midnight to file their returns and make payment or they will face a hefty fine.
Fines are automatically imposed for late filing
Nearly 900,000 taxpayers missed the deadline last year and received an automatic £100 fine as a result.
But it is hoped that fewer people will be fined this year because HM Revenue & Customs (HMRC) has sent out simplified returns to close to a million people.
However, one accountancy body told BBC News that it expected similar number of people to be fined this year as last.
An estimated 9.5 million people have received returns for tax year 2004/2005.
In addition to the £100 automatic fine, people who do not pay their tax bill by 31 January will also be charged interest at 7.5% on money owed.
WHO COMPLETES TAX RETURNS?*
Name or member of Lloyd's
Ministers of Religion (of any faith or denomination)
*Full list and exceptions on Revenue website
The HMRC will also impose a 5% levy if the debt has not been paid by 28 February, and another 5% on any money still owed on 31 July.
"This is the 10th year of self-assessment and the date shouldn't be a surprise," a HMRC spokeswoman said.
"If you do miss it you do get a penalty and have to pay interest on any tax not paid."
The HMRC anticipates record numbers of people will choose to file their tax returns online.
However, in previous years, the online filing system has been dogged by difficulties.
Last year, the HMRC extended the deadline for online filing after it emerged that a large number of taxpayers were unable to log on to file their returns.
To prevent a repetition, the Revenue has been urging people to file their returns at off-peak times, such as early in the morning or late at night.
In addition, the HMRC has been sending out shortened tax return forms of just four pages to people with simple affairs.
"If this does not lead to a fall in late filing - which it may not - then questions may be asked of the whole self assessment system, " Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants (ACCA) said.
"Every year we see the same numbers missing the deadline, there seems to be a cultural problem, with many people approaching self assessment as they do a visit to the dentists, putting it off and putting it off," he added.