The image depicts the king in profile facing to the left, known as a definitive stamp, using an adaptation of the portrait to be used on new UK coins in a tradition dating back to the creation of the Penny Black stamp for Queen Victoria in 1840.
The continuity also includes retaining the background colours of the stamps featuring Queen Elizabeth II: plum purple for first class, holly green for second class, marine turquoise for large first class, and dark pine green for large second class.
?The guidance we got from His Majesty was more about continuity and not doing anything too different to what had gone before,? said Royal Mail?s director of external affairs and policy, David Gold.
?There is no embellishment at all, no crown, just simply the face of the human being, on the plain background, almost saying: ?This is me and I?m at your service,? which I think in this modern age is actually rather humbling.?
Royal Mail said that in an effort to ?minimise the environmental and financial impact? of the change in monarch ? around 8bn letters are delivered in the UK each year ? retailers will exhaust their stocks of stamps featuring the late queen before selling the new ones.
?The King gave very clear directions ? he didn?t want anything to be pulped, he didn?t want things being shredded, he didn?t want stock being thrown away,? said Gold.
Charles is the seventh monarch to appear on a definitive stamp, sometimes referred to as an ?everyday? stamp, which only feature the monarch?s head and value of the stamp on a plain-coloured background.
?Ever since the Penny Black was issued in the reign of Queen Victoria, British stamps have carried the image of the reigning monarch,? Royal Mail?s chief executive, Simon Thompson, said.
?Uniquely, British stamps do not have the country of origin printed on them as the image of the monarch is sufficient. The definitive stamp has become a recognisable symbol of each reign.?