Dan Brown at a news conference. (Reuters)

London, March 19 (Reuters): A court case in which two historians accuse Dan Brown of copying their work in his novel The Da Vinci Code is due to finish tomorrow, ending one of the most closely watched copyright claims of recent years.

Michael Baigent and Richard Leigh wrote The Holy Blood and the Holy Grail, a work of historical conjecture published in 1982, which shares some of the same themes as Brown’s best-selling religious thriller.

They are suing Brown’s British publisher Random House, which also happens to be their own.

Both books raise the possibility that Jesus had a child by Mary Magdalene, she fled to France after the Crucifixion and Christ’s bloodline survives to this day. They also associate Magdalene with the Holy Grail.

Authors warn that should the historians succeed, there would be serious implications for fiction writers who have always incorporated other people’s ideas and research into their works.

Legal experts say the claimants face an uphill task to protect general ideas.

“You would hamper artistic creativity if you couldn’t write a novel that theorises about a conspiracy theory,” said Boston-based intellectual property lawyer Edward Naughton of Holland & Knight. “That’s why courts have been very wary about allowing protection of ideas that are this general.”

In his summing up arguments on Friday, Random House’s lawyer John Baldwin emphasised that ideas of a factual nature should be available for any reader to use.

Brown, 41, was present for most of the hearings and showed signs of frustration during three days of cross-examination. While he admits Holy Blood was one of his sources, he says he had many others and its ideas are not new anyway.

A Code character has a name that is an anagram of Baigent and Leigh, and he refers to their book in the narrative.

The claimants’ lawyer Jonathan Rayner James said Brown lied in his witness statement when he said neither he nor his wife Blythe, also his research assistant, had seen the Holy Blood before he wrote the synopsis for The Da Vinci Code in 2001.

Rayner James also questioned how Dan Brown knew the origin of his research when so much was carried out by his partner.

Although Blythe was not present at London's High Court, she emerged as a key figure in the case, not only researching for The Da Vinci Code but also coming up with some of its main themes including the sacred feminine and Jesus' bloodline.

With around 40 million copies of The Da Vinci Code sold and a Hollywood blockbuster starring Tom Hanks due for release in May, there is plenty at stake.

Sony Pictures plans to go ahead with the launch regardless, and legal experts say the case is only likely to affect the distribution of the movie in Britain should the claimants win.

The case is not the first time Brown has been accused of plagiarism. Last August he won a court ruling against Lewis Perdue, who alleged The Da Vinci Code copied elements of two of his novels,“Daughter of God” and“The Da Vinci Legacy”.

Publishing executives complain that the lawsuits are proof of the saying:“Where there's a hit, there's a writ”.