HM Revenue & Customs is warning individuals that if they have not filed their tax returns by 31 January, they could receive a £100 penalty.
To register online, visit www.hmrc.gov.uk/online, to receive a user ID and an activation code will be posted to you within seven working days. Once the self-assessment service has been activated, you can then file online.
HMRC said: "If you've filed online before, make sure that you have your user ID and password. Don't leave it to the last minute as it can take up to seven working days to get replacements if needed.
Visit www.hmrc.gov.uk/sa for help or contact the self assessment helpline on 0845 9000 444.