If Oxford-based medieval historian Karen Ralls was on TV's Mastermind, her special subject would be the Knights Templar. Knowing that they were surrounded by inaccurate myths, three years ago she wrote a book, The Templars and the Grail, to correct some of them. Two months later, a book by an author she had never heard of was released in the US.

Called The Da Vinci Code, its plot was fantastical, but it gripped people. They were dying to know if it could possibly be true. Because it involved the Knights Templar, the media started contacting her to check the historical facts and her book sold in increasing numbers.

Three years later, her book is on its tenth reprint, she is an old hand at dealing with the media and well used to people asking if the Templars actually found the Holy Grail. Does she see the attention as a double-edged sword?

"Oh yes, it is," she said. "While it is good that The Da Vinci Code phenomenon has created much more interest in the medieval Templars, I decry the sensationalism." She is quite firm about the facts. "We have no evidence that a Templar ever found a grail or wrote a grail romance." Nor did they build Rosslyn Chapel, where Dan Brown's book ends.

"The Templar order was suppressed in 1312 and Rosslyn Chapel wasn't started until 1450 by the St Clair family," she explained. "They were not Templars, because the order had long been suppressed." Karen should know. She spent several years as deputy curator there, while studying for a doctorate at Edinburgh University. A chapter of her book is devoted to the chapel.

The Knights Templar were initially warrior monks based in the Holy Land, who protected pilgrims on the way to the Crusades. They gradually spread throughout Europe, becoming trusted diplomats, farmers, business experts and navigators. "They were bankers to kings, popes, nobles and pilgrims, creating what is acknowledged today as the first western international corporation," she said. "They introduced a very early form of a 'letter of credit' system of travellers' cheques, in addition to the concept of a safe deposit box."

Answerable only to the Pope, they eventually became so powerful and influential that they were seen as a threat particularly by King Philippe IV of France, who set about plotting their downfall.

Her interest in the Knights Templar stems from researching her own family tree when she was a student. "One of our ancestors was a Brother William de la More," she said. "He was the last Templar master of the Temple in England. The other, William de Bernewood, was a leader of the London Templar preceptory in the early 13th century. So naturally, I became more interested in medieval Templar history."

d=3,3,1In her book, she looks at the known facts about the Knights Templar and addresses some of the myths. "Academics have a responsibility, an imperative responsibility to continue to separate fact from fiction," she said. She sees The Da Vinci Code as a gripping thriller, not a history book. However, she recognises the positive effects. "Students at many universities are switching majors to medieval history, art history or archaeology, all because of a novel," she said. "It has inspired many to read again, perhaps attend church again, or at the very least to take a more serious interest in medieval history and religious matters and that really is not a bad thing in the long run."

It has also raised the profile of a subject she loves. Her next two books are on the same topic. "One is an official Knights Templar encyclopedia, which will address the facts about the medieval order itself. The other work has to do with the importance of the medieval east-west trade routes," she said. She is cataloguing Templar sites and, through the recently formed Templar Heritage Trust, hopes to raise money to preserve some of them.

The Templars had a strong link to Oxford, with a large estate near Oxford, with headquarters first in Temple Cowley and later in Sandford. "The medieval Temple Cowley preceptory and its surrounding lands in Oxfordshire were very extensive indeed, far beyond where the borders of the modern-day shopping centre and the Temple Cowley swimming pool areas are today," Karen explained. "They even extended into what is now part of Oxford city, as well."

If only Dan Brown had known this, he could have set his denouement in Temple Cowley Library built on the site of the Templars' fish ponds or perhaps at Templars Square Shopping Centre. And the film could have ended at Oxford Castle, where the Knights were imprisoned when their Order was suppressed. What a missed opportunity for Oxford.

The Templars and The Grail is published by Quest at £18.99.