Two writers who have accused Dan Brown of breaching their copyright in his bestselling book The Da Vinci Code could face a half-million pound legal bill if they lose the case, a leading media lawyer said today.

David Hooper, a partner at the London law firm Reynolds Porter Chamberlain, said today that Michael Baigent and Richard Leigh face a difficult task in persuading the High Court that Brown went too far in using their 1982 book, The Holy Blood and the Holy Grail, as a source for his own novel.

Brown has never denied using the book as a source or inspiration, mentioning it in his novel and naming a central character Leigh Teabing in what has been seen as an anagrammatic tribute to Baigent and Leigh.

UK copyright law does not protect ideas, just the work in which they are expressed, and the two writers say that Brown copied the "architecture" or narrative structure of their work for his later novel.

As a result of their action, lawyers for both the two writers and for Random House, which published both books and is the actual target of the lawsuit, will have gone through both books with fine-toothed combs searching, respectively, for similarities and differences.

"That is a very expensive form of litigation because you're analysing and comparing one text with another and that takes many many hours work," Mr Hooper told Times Online.

"The Dan Brown book is a very quick read but the other one is harder work, especially if you're not interested in mysticism or religious topics. With appeals, this case has a £500,000 look about it."

He added: "It's like the Prince of Wales [and his case against The Mail on Sunday]. It's easy enough to start these things, you can find the precedents for them, but the problem is finding the evidence. They can get out of hand."

The success of The Da Vinci Code, which has sold more than 30 million copies worldwide and has been adapted for a film starring Tom Hanks and Sir Ian McKellen, has spawned dozens of academic works examining, and usually debunking, its central hypothesis, that Jesus had a child with Mary Magdalene whose descendants were protected by secretive Roman Catholic sects.

The idea that Jesus and Mary Magdalene may have produced a child first came to prominence because of the book by Leigh, Baigent and a third co-author, Henry Lincoln, who is not joining in the High Court action.

Their book was an airport bestseller in the 1980s, selling two million copies, and has enjoyed another boost since Brown's book was published in 2003.

Mr Hooper said that Brown, who arrived at the High Court today to take the stand in his defence, would argue that he and his wife had consulted many sources in writing his book. Brown's website says that there are thousands of sources referring to the central "secret" of his novel.

"Writers have always been able to work a theme. Take Shakespeare - his whole work worked ideas or histories that were around in his time. Baigent and Leigh do not have a monopoly on this idea," he added.

"Normally when people copy from books and commit plagiarism, they do it surreptitiously. But Dan Brown has been completely upfront about this. It may be that their book was the starting point, but as long as you don't actually copy it, then you are all right."

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