Just over three million people have yet to complete their 2016-2017 tax returns - with only one week to go before the January 31 deadline.
Around eight million people have already filed their self-assessment returns for 2016 to 2017, HM Revenue and Customs (HMRC) said.
The deadline for submitting 2016-2017 online self-assessment tax returns to HMRC, and paying any tax owed, is January 31.
The revenue body is reminding customers of a £100 penalty for sending late returns - even if there is no tax owed. Further penalties apply, depending on the lateness of the returns filed.
Angela MacDonald, HMRC director general of customer services, said: "The seven-day countdown to January 31 has now begun.
"Put a stop to that niggling feeling and do your self-assessment now."
Help is available from the gov.uk website at www.gov.uk/selfassessment.