The Royal Mail has brought in a new style of postage stamp which feature a barcode. You can read about it here: https://www.royalmail.com/sending/barcoded-stamps

Stamps which do not feature a barcode ceased to be valid on 31 July 2023 and should not be used. If they are used, the recipient will be charged a surcharge.

You can ‘swap out’ your old stamps for new ones and information on how to do this is on the above website. This applies to all ‘definitive stamps’ – the ones which feature only the Queen’s head in profile. This includes standard First and Second Class stamps, Large Letter stamps, and all specific value Definitive stamps (e.g. 1p, 2p, etc). Special Stamps with pictures on and Christmas Stamps without a barcode will continue to be valid and don’t need to be swapped out.

The pre-paid envelopes used by MPs’ offices will not be affected and can continue to be used and are not part of the swap out scheme.

Update 9 July 2024: the pre-paid envelopes bearing the head of Queen Elizabeth II are still valid, and there are currently no plans to replace them with the head of His Majesty King Charles III.