Dan Brown, whose The Da Vinci Code is claimed by the publisher to have become the bestselling hardback adult novel of all time, is facing possible legal action by the authors of a 1982 non-fiction bestseller and a 1983 novel.

Michael Baigent, Richard Leigh and Henry Lincoln — whose The Holy Blood and The Holy Grail was condemned by the Roman Catholic Church but continues to draw readers and disciples — are said to be preparing to sue him for alleged breach of copyright of ideas and research.

Another author, Lewis Perdue, is threatening to sue Brown for alleged plagiarism, claiming that he borrowed heavily from his novel, The Da Vinci Legacy.

The authors of The Holy Blood and The Holy Grail claimed to have found evidence that the Priory of Sion, a secret society founded in the late 11th century linked to the Knights Templars and whose grand masters supposedly included Leonardo da Vinci, Victor Hugo and Sir Isaac Newton, guarded documents that challenged orthodox Christian tenets and history.

The writers claimed that Jesus married Mary Magdalene and that their offspring fled the Holy Land and found refuge in a Jewish community in the south of France, thus preserving their lineage.

In The Da Vinci Code, Jesus did not die on the Cross, but married Mary Magdalene and started a family in France, where descendants of their child survive to this day. Leonardo was among members of the Priory of Sion, which guards the bloodline of Christ and Mary Magdalene, who knew the explosive secret.

Both narratives begin with a mystery that leads sleuths to more sinister intrigues. In The Da Vinci Code, it is the murder of Jacques Saunière, a curator of the Louvre. In The Holy Blood, there is the mysterious death of a real-life priest in the south of France, also called Saunière. Brown argues that the Da Vinci Code is a work of fiction, but, in a statement of fact prefacing his novel, he refers only to the documents in the Bibliothèque Nationale, “Les Dossiers Secrets”, which played a major part in the research done for The Holy Blood and The Holy Grail. Last month, The Washington Times wrote that Mr Brown “cribbed” most of his supposed historical facts from The Holy Blood and The Holy Grail. Ironically, the two books are published by the same publishing house, Random House, through the Corgi and Arrow imprints.

The respective agents for the three authors declined to comment yesterday on the legal action, but insiders at Random House confirmed that a legal case was being prepared.

The Da Vinci Code is to be made into a film, directed by Ron Howard and starring Harrison Ford.

Liz Thomson of Publishing News, said: “Charges of plagiarism and breach of copyright are notoriously hard to prove, but if Baigent and his co-authors — who have themselves now fallen out — do pursue a case through the courts, the action is likely to jeopardise Columbia’s planned film of The Da Vinci Code.”

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