Up to one million UK taxpayers could face escalating fines of up to £900 from HMRC. It was revealed last week that around 1 million Brits missed the January 31 deadline to submit their tax returns, which automatically triggered a £100 fine for each.

However, these fines can spiral out of control if left unchecked. Three months after the January 31 deadline, additional daily penalties of £10 per day apply, up to a maximum of £900. HMRC previously urged Brits to file their returns urgently.

Myrtle Lloyd, HMRC’s chief customer officer, said: “Thank you to the millions of people and agents who filed their self-assessment tax return and paid any tax owed by 31 January.”

The problems worsen if the deadline is missed by six months. A further penalty of 5% of the tax due, or £300, whichever is greater, will be charged. After 12 months, another 5% or £300 will be added, whichever is greater.

The scale of fines is substantial. HMRC raises tens of millions of pounds each year from penalties. However, the body said it will consider customers’ reasons for missing the deadline, and those with a reasonable excuse may be able to avoid being fined.

Millions of people have their tax automatically deducted from their wages through PAYE (pay-as-you-earn), but sole traders, self-employed people, or those who earn additional income may need to submit a self-assessment every year.

This includes anyone who earned more than £1,000 in the 2024-25 financial year through self-employment or by letting out a property or land.

The UK’s tax authority said 27,456 people filed their returns in the final hour before the cut-off for 2024-25 returns on Saturday, January 31. It received about 11.5 million submissions by the cut-off date.

The quickest way to resolve outstanding returns is via gov.uk or the HMRC app. Missing the deadline does not affect credit scores, and HMRC will not publicly disclose late filings.