Dan Brown, author of "The Da Vinci Code" faces an English High Court challenge from two men who claim he stole their ideas.
Michael Baigent and Richard Leigh are suing publisher Random House, claiming Brown's book draws heavily on their 1982 novel "Holy Blood, Holy Grail".
Brown's 2003 book, about a Vatican cover-up, has sold more than 30 million copies worldwide, earning the author $78.5 million in one year, by combining thriller, detective and conspiracy theory genres.
The novel has been translated into 44 languages and drawn criticism from the Roman Catholic Church and historians.
If Baigent and Leigh are successful and obtain injunctions preventing the use of their material it could threaten the British release of "The Da Vinci Code".
The adaptation, budgeted at $100 million, stars actors Tom Hanks, Sir Ian McKellen, Audrey Tatou and Jean Reno.
The case is likely to clarify the extent to which an author can use other people's research under existing copyright laws.
Brown apparently acknowledges the theories of "Holy Blood, Holy Grail" in his novel, by naming a character after the authors.
