Dr Marc Morris, an expert in the Middle Ages, said experts at the Royal Mint made a schoolboy error.
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, Morris said.
The charter enshrined that taxpayers had a right to be represented, the right to a fair trial and that nobody was above the law, even royalty.
The Royal Mint commissioned coin artist John Bergdahl to create the £2 (R35) coin to celebrate the 800th anniversary of the charter.
It is on sale for £10 (R176.65), complete with a medieval inspired display mount.
Morris, 41, the author of several books, among them King John: Treachery, Tyranny and the Road to Magna Carta, said the coin was historically wrong. “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.
“All the pen in John’s hand symbolises is ignorance of this basic fact.”
The Royal Mint defended the coin, saying the image was symbolic rather than factual.
– The Telegraph