An early edition of Magna Carta has been found in a Victorian scrapbook during a search of a council's archives
The discovery has come months ahead of the 800th anniversary of the sealing of Magna Carta in Runnymede in 1215.
Kent archivist Dr Mark Bateson had been asked to search for another charter from the town of Sandwich.
Dr Bateson found the town's Charter of the Forest in a Victorian scrapbook in Kent County Council archives - with the long-forgotten Magna Carta edition.
The document was ripped with about a third missing but could still be worth up to £10m, according to Professor Nicholas Vincent, a specialist in medieval history from the University of East Anglia.
Its high value comes from the fact that it was found with the Charter of the Forest. The only other such pair - dating from 1300 - in the world is owned by Oriel College, Oxford.
Professor Vincent, who asked Dr 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 said it backed the theory that Magna Carta was issued more widely than previously thought, to at least 50 cathedral towns and ports.
He also said the discovery gave hope that further copies would also turn up.
Twenty-four 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.
What was Magna Carta?
- Magna Carta outlined basic rights with the principle that no-one was above the law, including the king
- It charted the right to a fair trial, and limits on taxation without representation
- It inspired a number of other documents, including the US Constitution and the Universal Declaration of Human Rights
- Only three clauses are still valid - the one guaranteeing the liberties of the English Church; the clause confirming the privileges of the City of London and other towns; and the clause that states that no free man shall be imprisoned without the lawful judgement of his equals
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.
It is understood Sandwich does not plan to sell its copy but does hope to benefit from it as a tourist attraction.
Paul Graeme, mayor of Sandwich Town Council, said owning the forest charter and Magna Carta was "an immense privilege".
He said: "Through the American Declaration of Independence, continuing in the Universal Declaration of Human Rights, Magna Carta still underpins individual liberties worldwide.
"To own such a document - and the Charter of the Forest - is an honour and a great responsibility."
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.