They will not go on sale immediately because King Charles wants existing supplies of stamps adorning the late Queen to go out of stock first.
A key milestone of King Charles's reign has come as Royal Mail unveiled stamps bearing his profile.
The image of the King featured on first and second-class stamps has been revealed as a simple profile image, showing his head and neck in the style of the late Queen, and all British monarchs since Queen Victoria.
In keeping with stamp tradition, the King faces to the left - but unlike the Queen's stamp, he is not wearing a crown.
The same image, of a profile sculpture made by artist Martin Jennings, has been used by the Royal Mint to make coins. An image of the sculpture was adjusted and relit for use on the new stamps.
It's the first time his likeness is being featured on stamps since the death of his mother in September.
The world's first stamp - known as the penny black - was issued in 1840 and bore Queen Victoria's image.
The King guided Royal Mail to keep the style consistent and to use up existing Queen stamps rather than pulping them.
"The guidance we got from His Majesty was more about continuity and not doing anything too different to what had gone before," Royal Mail's director of external affairs and policy said.
The new 50p pieces represent the biggest change to UK coinage since decimalisation.
"I think perhaps there's an acknowledgement that, for 70 years people have been so used to seeing the image of Her Majesty, even though actually the current image only started in 1967, they didn't want to do anything too different to what had gone before," David Gold said.
Retailers will not start selling the new stamps until stocks featuring the late Queen are sold.
However, people can register interest in the new stamps now on the Royal Mail's website.
"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," Mr Gold said.
"He was very clear, however long it takes you to clear the stock, there's no rush."