A rare and lost original of the which has been tucked away in a library for years could be worth millions of pounds. The manuscript was one believed to be an unofficial "copy", yet is now thought to be a genuine version which makes it "one of the most valuable documents in the world".

In 1946, Harvard Law School paid $27.50, which would have equated to £7, and has since kept it hidden away. However, after further examination, two history professors have declared it to be the incredibly rare and lost Magna Carta from 1300, when King Edward I reigned. Digitised images of the document were published on the US university's website, which have been analysed by Prof David Carpenter from .

He concluded that the "fantastic discovery" is "the last Magna Carta".

"[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," Prof Carpenter said.

The academic expressed his amazement that an authentic version had been sold "for peanuts", as well as its legitimacy remaining undiscovered.

Upon acquiring the manuscript in 1946, Harvard filed it as HLS MS 172, describing it as a "copy made in 1327" which is "somewhat rubbed and damp stained".

The professors, who spent a year studying the document, believe it originated from the town of Appleby in Cumbria. They explain that the journey between the two can be traced back to the Lowther family who gifted the Magna Carta to Thomas Clarkson.

Mr Clarkson, a leading English abolitionist, passed down his estate which eventually ended up with the Maynard family. Air Vice Marshal Forster Maynard then sold the manuscript at auction at Sotheby's.

It was bought by a bookseller in London for £42, before Harvard acquired it later that year for a lower price.

"I would hesitate to suggest a figure, but the 1297 Magna Carta that sold at auction in New York in 2007 fetched $21m [about £10.5m at the time], so we're talking about a very large sum of money," Prof Nicholas Vincent said.

The Magna Carta is a charter signed by King John of England in 1215 which established the principle that the king and his government was not above the law.

The document was circulated across England, and was reissued by successive kings through to the year 1300. Prof Vincent said "there may have been 200 originals", 25 of which have survived.