In the United Kingdom, if you are required to file a tax return and you do not file it by the deadline, you may be subject to a late filing penalty. The amount of the penalty depends on how late the return is filed and how much tax is due.

If you file your tax return more than 3 months late, you will be charged a fixed penalty of £100. If you have not paid the tax that is due, you will also be charged interest on the unpaid amount.

If you file your tax return more than 6 months late, you will be charged a penalty based on the amount of tax that is due. The penalty is equal to the tax due divided by the number of days that the return is late, multiplied by the number of days that the return is late.

If you have a reasonable excuse for filing your tax return late, you may be able to appeal the late filing penalty. It is important to file your tax return on time to avoid incurring a late filing penalty. If you are having difficulty meeting the deadline, you can contact HM Revenue and Customs (HMRC) for assistance.