The Magna Carta would have been 'endorsed' by King John of England at Runnymede, near Windsor, on June 15, 1215, with a Royal seal, rather than with the stroke of a quill pen, said one of the historian.
The coin is on sale for 10 pounds, complete with a 'medieval inspired' display mount and is a celebration of the "catalyst towards freedom for the ordinary man".
"The coin shows King John signing the document - which was issued by him in 1215 and stated that everyone had the right to a fair trial and that nobody, including royalty, were above the law - with a quill but that notion is absurd as the monarch would have used a royal seal...And probably would have got one of his minions to do it," he said.
Medieval kings did not authenticate documents by signing them, they did it by sealing them - or rather by getting one of their officials to do it for them.
"The design is symbolic of King John's acceptance of the Magna Carta, it is not intended to be interpreted as a literal account of what actually occurred," Royal Mint spokesman said.
Royal Mint is a body that is permitted to manufacture coins of the UK.