Three new colours have been introduced for existing values in the Machin definitive range, beginning on January 3.

The 1st class and 1st class Large switched from gold to ‘Royal Mail red’, while the 50p became slate grey and the £1 wood brown.

All four values are self-adhesive, and in all cases the iridescent security overlay incorporates a year code, which is ‘M12L’ in the case of the 1st class, 50p and £1, and ‘MA12’ in the case of the 1st Large.

Codes correspond with the year in which the stamps are printed, rather than the year in which they are released, said a Royal Mail spokeswoman.