By Charlotte Beugge

Taxpayers who have to complete a self-assessment return for the 2010-11 tax year had better get their skates on.

All online tax returns need to be filed with HM Revenue & Customs (HMRC) by midnight on 31 Tuesday January if you want to avoid a penalty of £100 for late filing. If you owe tax for 2010-11 you also need to pay it by this date.

Even if you have no tax to pay or have already paid any owed, if you have been sent a return you still need to complete it by this date.

And it's now too late to file a paper return - they had to be in by 31 October - so you will have to file online. HMRC says that last year, more than 570,000 Brits actually left their filing until 31 January, making it by far the busiest day for the taxman last year.

HMRC says that while its systems can handle large volumes of online returns at busier periods, "it may take people a little longer to complete their online return".

If you do miss the deadline, then you will be penalised. If your return is three months late, then there will be additional penalties calculated at £10 for each day you are late up to a maximum of £900.

If you still haven't filed six months after the deadline, you'll be fined £300 or 5% of the tax due, whichever is the higher. For those who have not got their returns in after a year, there will be a further £300 or 5% fine and possibly, in serious cases, a fine of 100% of the tax due.