Brits who missed the Self Assessment tax deadline have been urged to act immediately – but not to call HMRC.

In a stark message on X (formerly Twitter), HMRC said anyone who has fallen behind should file their return online and pay any tax owed without delay.

The tax authority warned: “Missed the Self Assessment deadline? No need to call – Take action immediately to file your return online and pay any tax owed.”

The online service allows taxpayers to:

• File a Self Assessment if self-employed or if you receive other income, such as from renting out property
• Check previous returns, view what you owe, and print tax calculations
• Sign up for paperless notifications

HMRC also reminded first-time filers that they must register for Self Assessment before using the service. Anyone with an existing account should ensure it is active to avoid delays.

Certain taxpayers cannot use the online system, including partnerships, trusts, estates, non-resident individuals, and some other specialist cases. Commercial software or paper forms must be used instead.

To file, you’ll need your Unique Taxpayer Reference (UTR) and to sign in to the service. HMRC may also ask for ID, such as a passport or driving licence, to confirm your identity.

The warning comes as millions of Brits juggle work, bills, and taxes – with penalties for late submission starting at £100, rising quickly the longer returns are delayed.

Late payment penalties

Initial penalty: £100 fixed penalty immediately after the deadline, even if you owe nothing.

Three months late: Additional daily penalties: £10 per day, up to a maximum of £900.

Six months late: Further penalty: 5% of the tax you owe at this point.

12 months late: Another 5% penalty on the outstanding tax, plus possible additional penalties if HMRC considers the delay deliberate or careless.

HMRC also charges interest on unpaid tax from the due date until it’s paid. File your return online here.