John Matthews, who has devoted more than 30 years to the Arthurian legend, says that the film King Arthur, on which he has acted as an adviser, comes closer to the 5th-century ruler than any book.

Extensive research for the film convinced him that the Knights of the Round Table were exiled warriors and horsemen from Sarmatia, roughly modern-day Georgia.

In battle they fought under the leadership of Lucius Artorius Castus, who was born to a Roman father and a British mother: Artorius is Latin for Arthur.

Mr Matthews, who has written a number of books on Arthurian history, legend and literature, said: “There are interesting parallels between the Sarmatians and what we have known about Arthurian knights in medieval stories.” Their customs, gods and traditions included the practice of worshipping a sword stuck in a stone.

Arthur proved his superiority by drawing a sword from a stone and, when that broke, he got another, Excalibur, from the Lady of the Lake.

The weapon’s name may derive from a Sarmatian source in that an older name for it is Caliburn (white-steel), which in turn derives from “chalybus” (steel) and “eburnus” (white). The Sarmatians had a hero myth, Mr Matthews said. “One of their great heroes had his sword thrown into the water, just like Arthur.”

At the third attempt the sword goes beneath the water. Like the Knights of the Round Table, the Sarmatians were proud warriors who believed they had equal status.

They are known to have had a wind-sock banner shaped like a dragon which seemed to roar as they rode into battle. Arthur is thought to have had such a banner.

In later medieval and post-medieval accounts he is often called Arthur Pendragon, meaning “head of the dragon”.

The film expands on a theory first suggested in the 1920s. Some 3,000 Sarmatian warriors are known to have been sent to Britain as part of the Roman legions and were based in Cumbria.

Mr Matthews said: “When I was originally approached to advise on the movie I wasn’t convinced by the theory.

“I’ve ended up being convinced by the sheer weight of evidence.”

The film, starring Clive Owen with Keira Knightley as Guinevere, was made in Ireland from a script by David Franzoni, who wrote Gladiator.

It opens in Britain on July 30.

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