The Harvard Law School original copy

An unofficial copy of the Magna Carta, bought for seven pounds 80 years ago, could be worth more than £20 million.

Historians believe the document is an original version – making it one of the world’s most valuable manuscripts.

Harvard Law School bought the “copy” in 1946.

Since then, it has remained in its library.

Now, two professors of mediaeval history have deducted it is a rare and lost Magna Carta from 1327.

Professor David Carpenter, of King’s College London, said:

”This is a fantastic discovery.

“It is the last Magna Carta… [and it] deserves celebration, not as some mere copy, stained and faded, but as an original of one of the most significant documents in world constitutional history; a cornerstone of freedoms past, present and yet to be won.

“I am absolutely astonished that not only we have discovered this authentic version, but that, over the years, no-one seemed to know what they had and that it had been sold for peanuts.”

The Magna Carta is a charter issued by King John in 1215 that guaranteed the liberties and rights of his subjects.

It also put the Crown under the authority of the law.

More than 200 copies were made and sent throughout the country.

Professor Nicholas Vincent, of the University of East Anglia, Norwich, helped establish the Harvard document’s authenticity and provenance.

Magna Carta has influenced the framing of constitutions around the world.

Professor Vincent said:

“It is an icon both of the Western political tradition and of constitutional law.

”If you asked anybody what the most famous single document in the history of the world is, they would probably name Magna Carta.

“I would hesitate to suggest a valuation, but the 1297 Magna Carta was sold at auction in New York in 2007 for £10.7 million – so we’re talking about a very large sum of money.”