When will I need to pay HMRC the underpayment of tax?

You don’t need to directly pay HMRC, this will be done through your payroll. At the end of the tax year, HMRC recalculates the overall amount of tax due and issues a P800, which details how it calculated the underpayment.

To recover the underpayment of tax, HMRC will normally reduce an pensioner’s tax code in the following tax year (2027/2028) and/or subsequent year (2028/2029).