LONDON — Bought for $27.50 after World War II, the faint, water-stained manuscript in the library of Harvard Law School had attracted relatively little attention since it arrived there in 1946.

That is about to change.

Two British academics, one of whom happened on the manuscript by chance, have discovered that it is an original 1300 version — not a copy, as long thought — of the Magna Carta, the medieval document that helped establish some of the world’s most cherished liberties.

It is one of just seven such documents from that date still in existence.

“I never in all my life expected to discover a Magna Carta,” said David Carpenter, a professor of medieval history at King’s College London, describing the moment in December 2023 when he made the startling find.

The manuscript’s value is hard to estimate, although it is fair to say that its price tag of under $30 (about $500 today) must make it one of the bargains of the last century. A 710-year-old version of Magna Carta was sold in 2007 for $21.3 million.

Nicholas Vincent, a professor of medieval history at the University of East Anglia, in eastern England, helped authenticate the text. He noted that the document, which bound the nation’s rulers to acting within the law, had resurfaced at a time when Harvard has come under extraordinary pressure from the Trump administration.

“In this particular instance we are dealing with an institution that is under direct attack from the state itself, so it’s almost providential it has turned up where it has at this particular time,” he said.

Providential or not, the discovery happened largely by chance.

Carpenter was at home in Blackheath, southeast London, plowing his way through Harvard Law School’s digital images as research for a book when he opened a file named HLS MS 172 — the catalog name for Harvard Law School Manuscript 172.

“I get down to 172 and it’s a single parchment sheet of Magna Carta,” he said. “And I think, ‘Oh my God, this looks to me for all the world ... like an original.’ ”

Carpenter emailed Vincent, who was, at the time, at work in a library in Brussels.

“David sent it with a message saying, ‘What do you think that is?’ ” said Vincent. “I wrote back within seconds, saying, ‘You and I both know what that is!’ ”

The two academics were able to confirm the manuscript’s authenticity after Harvard Law School photographed it under ultraviolet light and subjected it to spectral imaging, a technique that can enhance aspects of historical documents undetectable to the human eye.

Amanda Watson, assistant dean at Harvard Law School’s library, said the document had sometimes been put on display. The library has yet to decide whether it will now be made available to the public.

The Magna Carta — “Great Charter” in Latin — was first issued in 1215. It has evolved into a global symbol of the importance of fundamental freedoms, including habeas corpus. By limiting the power of the monarch, it came to represent the right to protection against arbitrary and unjust rule.

The document influenced the U.S. Constitution, and the Bill of Rights includes several provisions that are thought to descend from the Magna Carta.