HM Revenue and Customs (HMRC) has issued a £100 fine warning to 12 million UK households who need to submit a Self Assessment tax return.

The tax office has warned that there is now just 100 days to go until the submission deadline on January 31, 2027, and is urging households to start their tax return well in advance to avoid the January rush. The January 31 deadline applies to people who file online, but if you are submitting a paper tax return, the deadline is 11.59pm on October 31, 2026, which is now just a week away.

But regardless of whether you file by paper or online, the deadline to submit your tax bill and pay what you owe for the tax year is 11.59pm on January 31, 2027, so there are still a few months for households to plan and budget for any tax owed.

According to HMRC, around 12 million people are expected to submit a tax return for the 2025 to 2026 tax year by January 31 and those who fail to file in time face being landed with a fixed £100 late filing penalty - even if there is no tax to pay.

HMRC says submitting tax returns early means households are able to find out how much tax they owe before the deadline, giving themselves more time to budget, and it helps to ensure tax returns are accurate, so you aren’t paying more tax than you need to.

Filing early can also mean households can get a refund quicker if they’ve paid too much tax and crucially, it means avoiding a penalty for submitting your tax return late.

In a reminder today, October 23, Myrtle Lloyd, HMRC’s Chief Customer Officer, said: “The Self Assessment deadline is 100 days away so now is the ideal time to get ahead and file your tax return early.

“Guidance and support are available on GOV.UK to help you file your tax return. Just search ‘file my Self Assessment’ on GOV.UK to start yours today.”

You must send a tax return to HMRC if, in the last tax year from April 6 to April 5, any of the following applied:

You may also need to send a tax return if you have any untaxed income, such as money from renting out property, tips and commission, savings interest, dividends or foreign income.

Failure to submit your tax return and pay any tax owed by January 31 will result in a fixed £100 late filing penalty, even if you don’t have any tax to pay.

After three months those who still haven’t filed their tax return will face an additional penalty of £10 per day, up to a maximum of £900. This will increase yet again after six months with a further penalty of 5% of the tax that is due, or £300 – whichever is higher. After 12 months, another 5% – or £300 – will be added.

If the tax remains unpaid after the deadline, interest will also be charged on the amount that is owed, in addition to the extra penalties.

HMRC added: “Check the filing deadlines. The deadline for sending a paper return is before the deadline for an online return. You can send your return any time after 5 April. Sending it earlier means you:

• find out what you owe

People who are new to Self Assessment must first register to receive their Unique Taxpayer Reference, which they will need when they complete and file their return, and anyone who needs to tell HMRC that they no longer need to submit a tax return can do so on GOV.UK.

Those signed up to Making Tax Digital for Income Tax still need to submit their Self Assessment tax return in their usual way and pay any tax owed by January 31, 2027.

Self Assessment customers with a PAYE income can file online before December 30, 2026, and have any tax owed collected via their tax code.