If you do not pay your taxes on time to the Her Majesty's Revenue and Customs (HMRC) in the United Kingdom, you may be subject to late payment penalties. These penalties can be applied to both underpaid and unpaid taxes.

The amount of the penalty will depend on how late your payment is and how much tax you owe. For example, if you pay your taxes late but within 30 days of the due date, you may be charged a penalty of 5% of the unpaid tax. If you pay more than 30 days late, the penalty increases to 5% of the unpaid tax for the first 30 days and an additional 5% for each subsequent 30-day period.

Additionally, interest will be charged on any unpaid taxes from the original due date until the date of payment.

It's important to note that if you have a reasonable excuse for not paying your taxes on time, you may be able to have the penalty waived by the HMRC.