An intriguing, previously unknown 13th-century version of a tale featuring Merlin and King Arthur has been discovered in the archives of Bristol central library.

The seven handwritten fragments of parchment were unearthed bound inside an unrelated volume of the work of a 15th- century French scholar.

Written in Old French, they tell the story of the Battle of Trèbes, in which Merlin inspires Arthur’s forces with a stirring speech and leads a charge using Sir Kay’s special dragon standard, which breathes real fire.

The fragments are believed to be a version of the Estoire de Merlin – the story of Merlin – from the Old French sequence of texts known as the Vulgate Cycle or the Lancelot-Grail Cycle.

Other versions of the cycle are known but this one features subtly different details. For example, different characters are responsible for leading the four divisions of Arthur’s forces.

In addition, in other versions, Arthur and Merlin’s enemy, King Claudas, is wounded on the thigh, while in the newly discovered fragments the nature of the injury is not specified. This may lead to a new interpretation of the text as upper-leg injuries are often used as metaphors for impotence or castration.

Dr Leah Tether, the president of the British branch of the International Arthurian Society and an academic based at Bristol University, cautioned that much more work needed to be done but said it was possible that these pages could be part of the version of the Vulgate Cycle that Sir Thomas Malory used as a source for his work Le Morte D’Arthur – which is the inspiration for many modern retellings of the Arthurian legend.

A detail showing the name Merlin in a handwritten parchment fragment of a manuscript from the middle ages. Photograph: University of Bristol/PA

Scholars are not convinced that Malory used any of the versions that until now had been known about.

Tether said: “These fragments are a wonderfully exciting find, which may have implications for the study not just of this text but also of other related and later texts that have shaped our modern understanding of the Arthurian legend.

“There is a small chance that this could be connected to a version that Malory had access to but we are a long way from proving that.”

If the fragments were printed in a modern paperback they would fill about 20 pages. There has been some damage to them, which means it will take time to decipher all the text.

“Time and research will reveal what further secrets about the legends of Arthur, Merlin and the Holy Grail these fragments might hold,” Tether said.

“The south-west of England and Wales are, of course, closely bound up with the many locations made famous by the Arthurian legend, so it is all the more special to find an early fragment of the legend – one pre-dating any version written in English – here in Bristol.”