The discovery of the version of the historical parchment which established the principle of the rule of law, in the files of the history department of Kent county council.
The document was found in the archives kept in Maidstone but belonging to the town of Sandwich.
The document was ripped with about a third missing but could still be worth up to 10 million pounds, according to Professor Nicholas Vincent, a specialist in medieval history from the University of East Anglia.
Magna Carta is one of the most important, well-known documents in history. It is considered one of the first steps towards parliamentary democracy and includes the principle that no one was above the law, including the king.
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The Magna Carta was signed on June 15, 1215 at Runnymede. The document was agreed by King John to appease rebel barons in the heart of battle.
Prof Vincent, who asked Bateson to search for the forest charter in December and went on to authenticate the Sandwich Magna Carta after it was found, said it was "a fantastic discovery".
He also said the discovery gave hope that further copies would also turn up.
24 editions of Magna Carta, which established the principle of the rule of law, are currently known to exist.
The Magna Carta Research Project describes the Sandwich document as a "previously unknown exemplar of the 1300 Magna Carta", over half a metre in length, but missing text and its royal seal.
Experts said the 1300 Magna Carta was issued by Edward I and was apparently the last to be drawn up. They said the discovery of the Sandwich Magna Carta brought to seven the number of surviving originals of 1300.
Paul Graeme, mayor of Sandwich Town Council, said owning the forest charter and Magna Carta was "an immense privilege".
This week, four 1215 versions were brought together at the Houses of Parliament; two from Salisbury Cathedral and Lincoln Cathedral and two from the British Library.