A "copy" of the Magna Carta, the medieval English document that has formed the basis of constitutions around the world, owned by Harvard Law School is actually an exceedingly rare original, British researchers said on Thursday.

Experts from King's College London and the University of East Anglia said the document, which the US institution acquired in the 1940s for US$27.50, is just one of seven from King Edward I's issue of Magna Carta in 1300 that still survive.

The Magna Carta is seen as a precursor of democracy and the basis of legal systems across the world as well as human rights conventions.

Dating from June 1215, the Great Charter – considered one of the world's most important documents – first put into writing the principle that the king and his government were not above the law.

It was a key document in the drawing up of the US Declaration of Independence and constitution.

King Edward I then issued a final version of the Magna Carta with minor changes, also known as the Confirmation of the Charters, in 1300.

It was a reaffirmation of the original Magna Carta signed by King John and the last full issue of the document.

In 1946, the Harvard Law School Library bought what it thought was a copy for US$27.50, the equivalent of just over US$470 today, according to a US Department of Labor inflation calculator.

But researchers noted that its dimensions were consistent with those found in the six previously known originals, as is the handwriting, with the large capital "E" at the start in "Edwardus" and the elongated letters in the first line.

"This is a fantastic discovery," said David Carpenter, professor of medieval history at King's College London.

"Harvard's Magna Carta 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." (AFP)