The British release of The Da Vinci Code, due on May 19, could be delayed or even halted if a copyright claim by the authors of a non-fiction book, The Holy Blood and the Holy Grail, is upheld.

Michael Baigent and Richard Leigh, who co-wrote the book 22 years ago, claim breach of copyright on the ground that the “architecture” or complex structure of their book — its essential theory — was plagiarised in Dan Brown’s The Da Vinci Code.

And next week the reclusive multimillionaire author will step out of the shadows into a courtroom. At stake is the vast fortune he has made from his worldwide cult hit and the millions more he will make from the eagerly awaited film.

Brown will be in London to defend his blend of conspiracy theory, sinister secret societies and thrilling adventure from the ground-breaking copyright claim whose significance could far outstrip its estimated £10 million worth.

He will be the key defence witness in an action brought against Random House, the parent publishers. The claim has the potential to set a precedent in copyright law, over the extent to which one author can draw on another’s ideas.

If the judge upholds the claim, Baigent and Leigh could seek an injunction preventing further infringement, affecting further sales of the book and delaying release of the film. Alternatively they could seek a share of profits.

The authors argue that Brown’s thriller, a story of church conspiracy and murder, draws its basis from the facts and theory of their book, researched over ten years. They say that Brown, a former English teacher from New Hampshire, used their central hypothesis, that Jesus Christ married Mary Magdalene and had a child, founding a bloodline that was protected down the ages by secret societies such as the Knights Templar.

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.

The authors will point to numerous specific references in Brown’s book, including the character Leigh Teabing, whose name, they say, is a tribute to themselves: Leigh from Richard Leigh and Teabing an anagram of Baigent.

One of the characters also finds a copy of The Holy Blood and the Holy Grail on Leigh Teabing’s library shelves.

Brown, who lives with his art historian wife in New Hampshire, seldom grants interviews, confining most communications to his website. The author, whose career earnings are estimated to be close to £200 million, has stopped taking commercial flights because of autograph hunters.

He has spoken in the past of his childhood surrounded by the secrecy of the clandestine clubs of Ivy League universities and Masonic lodges.

Random House will argue that Brown’s book drew on a wide variety of sources. But Baigent and Leigh, respectively a New Zealander and an American who lives in London, argue that they filed proceedings soon after publication of the book in Britain. They say that anyone who reads both books will be struck by the core similarities.

A third co-author, Henry Lincoln, is unwell and has decided to remain out of the action. Copyright law protects the expression of an idea rather than the idea itself. So the High Court action will test where the line of protection can be drawn and whether it will include a hypothesis.

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