Proactive Investors - Royal Mail (LON:IDSI) released images of stamps featuring King Charles III which will be available to buy from 4 April.
According to the Royal Mail website, the image is an adapted version of the portrait created by Martin Jennings for the Royal Mint, which will be used on new coins.
King Charles is facing left, known as the definitive stamp, and is not wearing his crown.
First-class stamps will be a plum purple colour, second-class holly green, first-class large marine turquoise, and second-class large dark pine green.
Royal Mail, which is now owned by International Distributions Services, said existing stamps featuring the late Queen Elizabeth II will be distributed and issued as planned and will remain valid for use.