An edition of the Magna Carta which could be worth up to STG10 million ($A19.65 million) has been found after it lay forgotten in a British council's archives.

The discovery of a version of the English document which established the principle of the rule of law, was uncovered in the files of the history department of Kent County Council.

It was found in the archives kept in Maidstone but belonging to the town of Sandwich.

Professor Nicholas Vincent, of the University of East Anglia, who authenticated the document, said: "It is a fantastic discovery which comes in the week that the four other known versions were brought together at the Houses of Parliament."

Prof Vincent said the fact Sandwich had its own Magna Carta gives backing to the theory that it was issued more widely than previously thought to at least 50 cathedral towns and ports.

There are only 24 editions of the Magna Carta in known existence around the world.

The first Magna Carta was drafted by the Archbishop of Canterbury and agreed by King John on June 15, 1215 to make peace with a group of rebel barons.

Considered the cornerstone of freedom, modern democracy, justice and the rule of law, the document forms the basis for legal systems across the globe, the Universal Declaration of Human Rights and the US constitution.