January 4 2010.

HM Revenue and Customs (HMRC) is issuing a timely reminder to anyone who hasn't yet submitted their 2008-09 Self Assessment tax return - do it online, by the end of January, or you could risk a penalty.

The 31 October deadline for paper returns has now passed, which means you must file your return online by the 31 January deadline. If you submit a paper return after 31 October, or you file online after 31 January, you could receive a £100 penalty.

If you're a first-time Self Assessment filer, or you are new to online filing, you'll first need to register for HMRC's Self Assessment Online service. Registering is simple - visit the HMRC website at www.hmrc.gov.uk/sa and click "Register for Self Assessment Online". Once you've registered, an Activation Code will be sent to you in the post - it may take up to seven days for this to arrive, so it's advisable to register as soon as possible.

When filing online, your tax is calculated automatically, you get an online acknowledgement once you've filed, and it's processed faster, so any money you're owed is repaid more quickly.

HMRC's Sarah Walker said: "If you haven't yet sent in your 2008-09 tax return, you need to start thinking about it now. And if you haven't filed online before, allow plenty of time to complete the registration process - that way, you'll avoid a last-minute rush at the end of the month."

And don't forget, 31 January 2010 is also the deadline for paying any tax you owe for the 2008/09 tax year.

Help is available from the HMRC website at

Additional help is available from the Self Assessment helpline on 0845 9000 444, which is open seven days a week from 8am to 8pm.