STORY: From coins and banknotes and to the official royal cypher used by the government, Britain has been slowly introducing replacements featuring the new monarch since his mother's death in September.
In keeping with a tradition dating back to the first Penny Black in 1840, the new "definitive" stamp uses an adapted version of a portrait of Charles by British sculptor Martin Jennings which is also appearing on new coinage.
"As with all stamps, the monarch approved them and so we hope that he's happy with this design," said David Gold, Director of External Affairs & Policy at the Royal Mail.
"The guidance we were given was not to try to be too clever or to try to veer off into some different direction, but very much to keep that traditional image that we're all very much used to."
The new stamp, which will go on general sale at the start of April, consists solely of the king's head and its value on a plain coloured background.
"There is precedent for the king not wearing a crown," Gold said.
Charles is the seventh British monarch to appear on a definitive stamp. Existing stamps with Elizabeth's image will remain valid and in distribution until stocks are exhausted, the Royal Mail said.