The letters will go out in the next six weeks targeting 560,000 pensioners who do not complete annual self assessment returns or where HMRC cannot automatically deduct the tax owed using a PAYE tax code.

This means that for the first time 140,000 pensioners will face a new tax bill for the 2023-24 tax year.

Highlighting the scale of the problem, and concerns about awareness among pensioners about their tax liability, HMRC has produced an online guide explaining how to pay the tax through the ‘simple assessment’ system.

However, the guidance does not include a phone helpline and directs pensioners to use online services on HMRC's website or the HMRC app to make payments. This raises significant concerns about how this will work as many pensioners will not be digiterally literate and will need support from family, friends and accountants.