A little-known medieval poem written almost 800 years ago by Scottish borders monks was revealed on Thursday as the earliest independent account of one of the single most important events in English history: the sealing of Magna Carta.

Curators at the British Library have been researching all aspects of Magna Carta for an exhibition marking its 800th anniversary next year.

But even they, said the library’s chief executive Roly Keating, were surprised by what they found in the Melrose Chronicle, written in the 13th century by worldly Cistercian monks. “It is unmistakably an account of what happened,” Keating said. “It is a measure of how, even after 800 years, the plot continues to thicken around Magna Carta.”

At Runnymede, beside the Thames, King John on 15 June 1215 met aggrieved English barons who had backed his failed war with the French. They demanded change and got it with Magna Carta, a document that established a new relationship between a king and his subjects.

Since then it has been interpreted and misinterpreted endlessly but – with its clause “to no one will we sell, to no one deny or delay right or justice” – remains a touchstone for civil liberties.

The Melrose poem, written in Latin, is remarkably clear. It begins: “A new state of things begun in England; such a strange affair as had never been heard; for the body wishes to rule the head, and the people desired to be masters over the king.”

It goes on to explain the anger at King John. “The king, it is true, had perverted the excellent institutions of the realm, and had mismanaged its laws and customs, and misgoverned his subjects. His inclination became his law; he oppressed his own subjects; he placed over them foreign mercenary soldiers, and he put to death the lawful heirs, of whom he had obtained possession as his hostages, while an alien seized their lands.”

The library’s curator of medieval manuscripts, Julian Harrison, who made the discovery, said the Melrose Chronicle had simply never figured on anyone’s radar.

“It does set out in quite precise terms the sequence of events at Runnymede for negotiations between the king and the barons. There is no official report of what happened. We don’t actually know who was present. This is sufficiently detailed to suggest that the person was present or knew somebody who was present.”

Harrison’s hunch is that it may have been someone in the retinue of Alan of Galloway, a Scottish nobleman who is known to have been at Runnymede, who imparted the information. A scenario can be imagined of Alan and his men heading home and stopping off at Melrose rest and telling the monks of the strange events in England.

Whatever the true story, the chronicle will be an important part of next year’s Magna Carta Show, to be sponsored by Linklaters, which will be the largest of the celebrations throughout Britain. It will include, said Keating, “some of the most precious loan items we’ve ever brought under the roof here” – not least from the US, the Delaware copy of the Bill of Rights and Thomas Jefferson’s handwritten copy of the Declaration of Independence.

There will be artefacts and letters from across the centuries exploring the enduring potency of Magna Carta including one from a British civil servant in 1947 detailing a decision not to endorse a plan for a Magna Carta celebration in Commonwealth countries. The reason? It might inspire “uncritical enthusiasm” for Magna Carta among “ill-disposed colonial politicians” in their struggle to acquire greater freedom for their peoples.

As well as the exhibition the library has commissioned a work from artist Cornelia Parker which responds to the legacy of Magna Carta in the digital era.

Parker plans to create a 13-metre long tapestry replicating Magna Carta’s Wikipedia page as it appeared in June this year. It will be stitched by over 200 people who have an association with Magna Carta including lawyers, prisoners, civil rights campaigners, politicians and barons. Jimmy Wales, the founder of Wikipedia, has also agreed to do some stitching.

Parker said she wanted to echo the communal activity that resulted in the Bayeaux Tapestry “but on this occasion placing more emphasis on the word rather than the image”.

• The photograph on the article was changed on 28 November 2014. The earlier photograph was captioned as being the Melrose Chronicle but was not.