The American author Dan Brown faced a High Court claim today that the central theme of his blockbuster novel The Da Vinci Code violated the copyright of an earlier book.

Michael Baigent and Richard Leigh are suing their own publishers, Random House, claiming that Brown's international bestseller lifts the "complete architecture" from their 1982 book, The Holy Blood and the Holy Grail, including the central hypothesis that Jesus and Mary Magdalene had produced a secret child.

The case raises the unlikely question of whether an author can borrow someone else's well-worn conspiracy theory and still be accused of breach of copyright. The reclusive Brown arrived at the Royal Courts of Justice today to defend his reputation, but is not expected to testify until next week.

Jonathan Rayner James, QC, representing the authors of the Holy Blood book - referred to in court as HBHG - told Mr Justice Peter Smith that The Da Vinci Code (DVC) is an infringement of his clients’ copyright.

Mr Rayner James said: "The claimants are not alone in this. Many people all over the world have commented to the same effect since The Da Vinci Code was first published."

He said that Brown claims the Holy Blood was "incidental" to the creation of his book and was consulted only at the very end of its making. "This is an extraordinary claim that would surprise anyone who has read The Da Vinci Code after reading the HBHG."

Mr Rayner James said that the HBHG was "historical conjecture" setting out the authors’ hypothesis. "The authors’ historical conjecture has spawned many other books that developed aspects of this conjecture in a variety of directions. But none has lifted the central theme of the book."

He said the authors had invested a "massive amount of their lives" researching the HBHG between 1976 and 1981. "There can be no dispute that Brown was aware of the importance of the HBHG to the central theme when he wrote DVC."

Mr Rayner James said that Brown's Sir Leigh Teabing character - seen as an anagram of the earlier authors' surnames - even mentions the importance of HBHG in the novel.

But Brown had said that the HBHG was not "crucial or important" to the creation of the central theme of his novel and when he wrote his synopsis, he had not even read it.

The American author has said that he had not even heard of HBHG until he saw it mentioned in some of his research books. "This cannot be correct," said Mr Rayner James.

The non-fiction work HBHG posits the theory that Jesus and Mary Magdalene married, had a child whose blood line continues to this day and moved to France, where they were hidden by the Catholic Church.

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