Around 5.7 million people have less than a month to submit their tax return or face a fine, according to HM Revenue and Customs (HMRC).
By 31 January, more than 12 million taxpayers are expected to file a return for the 2021/22 tax year.
People who miss the 31 January deadline could face an initial £100 fixed penalty, even if there is no tax to pay, followed by further charges.
Myrtle Lloyd, HMRC's director-general for customer services, said: "There is less than one month for customers to submit their tax returns and my message to those yet to start is: don't delay, do it online.
"HMRC provides lots of useful information to help you get started. Visit gov.uk and search 'self assessment'."
The revenue body said it will treat those with genuine excuses leniently, as it focuses on those who persistently fail to complete their tax returns and deliberate tax evaders.
Customers who provide HMRC with a reasonable excuse before the January deadline can avoid a penalty after this date.
Thousands chose to do it on the cusp of 2023, with some 42,500 people submitting their return on New Year's Eve or New Year's Day.
People should also watch out for scammers using the forthcoming deadline to trick people with fake texts, calls and emails, which may claim that tax is owed or that people are entitled to a tax "rebate".
People who miss the 31 January deadline could face an initial £100 fixed penalty, even if there is no tax to pay, followed by further charges.
Myrtle Lloyd, HMRC's director-general for customer services, said: "There is less than one month for customers to submit their tax returns and my message to those yet to start is: don't delay, do it online.
"HMRC provides lots of useful information to help you get started. Visit gov.uk and search 'self assessment'."
The revenue body said it will treat those with genuine excuses leniently, as it focuses on those who persistently fail to complete their tax returns and deliberate tax evaders.
Customers who provide HMRC with a reasonable excuse before the January deadline can avoid a penalty after this date.
Thousands chose to do it on the cusp of 2023, with some 42,500 people submitting their return on New Year's Eve or New Year's Day.
People should also watch out for scammers using the forthcoming deadline to trick people with fake texts, calls and emails, which may claim that tax is owed or that people are entitled to a tax "rebate".