Millions of UK workers could be in danger of missing a crucial deadline and incurring penalties from HM Revenue and Customs (HMRC). The majority of people required to complete a Self Assessment must pay the tax they owe by 11.59pm on 31 January 2026.

Those who need to submit an online tax return must also adhere to this deadline or face a late submission penalty. Many people unfamiliar with the Self Assessment system will require their Unique Taxpayer Reference to correctly file for the 2024/25 tax year.

Remember, this deadline only applies to those submitting an online tax return. If you were required to submit a paper tax return, it should have been submitted by 11.59pm on 31 October 2025 – if not, you would already be facing a penalty for late filing.

Fines for missing the tax deadline

HMRC can impose a financial penalty for any late tax returns. Starting with a fixed £100 charge, this can quickly increase if ignored.

If your tax return is not filed within three months of the deadline, additional penalties of £10 per day will be incurred, up to a maximum of £900. After six months, this penalty increases to five per cent of the tax owed or £300, whichever is higher.

After a year, an additional charge of 5% or £300 will be added – again, whichever is the larger amount. HMRC emphasises that these penalties can be easily avoided by simply submitting your Self Assessment tax return on time.

Furthermore, those who fail to pay their tax on time will initially be charged 5% of the unpaid tax 30 days after the due date. This penalty will then increase by an additional 5% if the payment is six months and 12 months late, respectively.

HMRC has also emphasised that interest will be levied on any outstanding tax owed.

What if I receive a penalty?

If you are issued a penalty by HMRC for either a late tax return or a late payment, you may be able to contest it if you disagree with the decision made by the government department. According to HMRC guidelines, one valid reason for disputing the penalty is if you have a ‘reasonable excuse’.

Typically, you have a 30-day window from the date the penalty is issued to contact HMRC and formally lodge an appeal. If a deadline was missed, you will need to provide a justification for the delay.

The method of making the appeal can vary greatly depending on the type of tax you pay and whether you’re employed or self-employed. Comprehensive details regarding this can be found here.

Typically, you have a 30-day window from the date the penalty is issued to contact HMRC and formally lodge an appeal. If a deadline was missed, you will need to provide a justification for the delay.

The method of making the appeal can vary greatly depending on the type of tax you pay and whether you’re employed or self-employed. Full details of this can be found here.