HM Revenues and Customs (HMRC) is reminding people to file their tax returns ahead of the October 31 deadline - or face a £100 fine.
Paper returns received on or after November 1 will result in penalties, even if there is no tax to pay or the tax due is paid on time.
This follows the introduction of a new penalty regime this year and the longer people delay, the bigger the fine.
There will be further late-filing penalties after three, six and 12 months.
Tax returns can also be filed online however if paper returns are received after October 31, initial £100 fines will still be issued even if information is subsequently filed online.
The deadline for online returns is January 31 and tax is calculated automatically with immediate acknowledgements.
For help and advice on completing a return, visit www.hmrc.gov.uk/sa or call the Self Assessment helpline on 0845 9000 444.