Richard Leigh and Michael Baigent are suing their own publisher Random House for lifting "the whole architecture" of the research that went into their 1982 non-fiction book The Holy Blood, and the Holy Grail, itself a bestseller.

Brown's religious thriller has more than 36 million copies in print worldwide and has upset Catholics for suggesting Jesus married Mary Magdalene and had a child by her. The same theory is put forward in The Holy Blood, and the Holy Grail.

Lawyers on both sides of the case have declined to comment on how the trial might affect sales of the hugely successful novel or the distribution of a major Hollywood adaptation, which Sony Pictures plans to release in May.

Jonathan James, representing Baigent and Leigh, told the High Court, "Dan Brown copied from The Holy Blood, and the Holy Grail and therefore the publication of the result by the defendant is in infringement of the copyright of my client in the United Kingdom."

Random House, owned by German media conglomerate Bertelsmann, have dismissed the claim as "without merit," and succeeded last October in having a "substantial" part of the case made by the historians dropped.