Smash hit

Brown's book, which has sold more than 30 million copies, is based on a similar hypothesis, except that in his work Jesus does not survive the crucifixion and Mary Magdalene brings up the child alone.

Speaking outside the High Court, Brown, who describes himself as a committed Christian, said that the distinction was an important one. "Suggesting a married Jesus is one thing, but questioning the Resurrection undermines the very heart of Christian belief," he said.

If the two writers are successful and opt to take injunctions stopping use of their material, it could threaten the British release of the film adaptation of the novel, starring Tom Hanks and Sir Ian McKellen, which is scheduled to open on May 19.

The case, expected to last up to two weeks, is also likely to clarify existing copyright laws over the extent to which an author can use other people’s research.

Gail Rebuck, chief executive of Random House which published both books, said: "We are genuinely saddened that two of the three authors of HBHG have chosen to bring litigation against us.

"Random House takes no pleasure in defending legal action that it believes is without merit and we are confident that we shall prevail."

John Baldwin, QC, representing Random House, said in papers put before the London court that HBHG had generated controversy when it was published over the idea that Jesus survived the Crucifixion and founded a bloodline that continues to the present day.

"In brief, the complaint appears to be that HBHG discloses the idea that Jesus was married to Mary Magdalene, that they had children which survived and married into a line of French kings, that the lineage continues today, and that there is a secret society based in France which has the objective of restoring this lineage to the thrones not only of France but to the thrones of other European nations as well, and that DVC uses some of this idea."

He said that there is no copyright in information of this nature and there had been no copying from HBHG. Mr Baldwin said many of the ideas complained of were not even in both books and most were not original to HBHG anyway.

David Hooper, a copyright specialist and partner at the London law firm Reynolds Porter Chamberlain, said that there were some precedents for non-fiction writers who claimed that creative artists plagiarised their works.

In 1980, Trevor Ravenscroft successfully sued James Herbert of his novel The Spear, which was based on a book written by Ravenscroft about the spear used to pierce Christ's side at the crucifixion.

In that case there were numerous alleged instances of verbal plagiarism, although the court did rule that Herbert had breached copyright by using the underlying structure and interpretation of events.

Mr Hooper said: "The problem here is not just that Baigent and Leigh do not have a monopoly on this idea, but that I don't think there is the copying."

Hooper said that he expected Leigh and Baigent to argue not so much that Brown borrowed large chunks of text, but that an examination of the books, their structure, and the description of key events involved "a second tier of copying".

"But that is a very difficult thing to prove, because there is no monopoly of an idea. We can all read a novel and use that to go off and write our own film scripts providing we don't copy it."

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