Self Assessment tax returns must be submitted online via the HM Revenue & Customs website by midnight on Monday 31st January 2011 in order to avoid an automatic penalty of £100.
The deadline is only later than this if you received your tax return, or the letter telling you to complete a tax return, after 31st October.
If HMRC receives your tax return after the 31st January, you'll be charged an automatic £100 penalty!
If a Partnership Tax Return is late there's a £100 penalty for each partner.
If a Trust and Estate Tax Return is late there's a £100 charge to the trust or estate.
You must pay any amount due for 2009-10 by 31st January 2011. This payment deadline is the same whether you file on paper or online.
HM Revenue & Customs will now allow you to pay your tax by Direct Debit if you've registered for Self Assessment Online. Direct Debit allows you to set up a payment as soon as you've worked out how much you owe - so you never need to be late with your payment.
For all the latest information and advice, take a look at the HM Revenue & Customs: Self Assessment website.
Read what other young entrepreneurs are saying about Self Assessment in the Shell LiveWIRE Discussion Forum.