A claim that Dan Brown's bestselling novel The Da Vinci Code copied the ideas of two other authors has gone before London's High Court.
Dan Brown's book has sold more than 30 million copies worldwide
Michael Baigent and Richard Leigh say Mr Brown stole "the whole architecture" of research that went into their 1982 book The Holy Blood and the Holy Grail.
Both books contain the idea Jesus had a child. They are suing publisher Random House, which denies the allegation.
Mr Brown attended the court on Monday as a judge heard initial submissions.
The Da Vinci Code won best book at this year's British Book Awards and more than 30 million copies have been sold in about 40 languages.
It has been adapted into a $100m (£57m) movie starring Tom Hanks, Audrey Tautou and Sir Ian McKellen, which is due to open this year's Cannes film festival.
Mr Baigent and Mr Leigh's non-fiction best-seller The Holy Blood and the Holy Grail (HBHG) was also published by Random House.
It dealt with theories that Jesus and Mary Magdalene married and that their blood line continued to the present day.
Richard Leigh (left) and Michael Baigent are suing Random House
Their lawyer Jonathan James told the 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."
Mr Baigent and Mr Leigh wrote their book with a third author, Henry Lincoln, who is not taking part in the claim due to ill-health.
Mr James added: "The authors' historical conjecture has spawned many other books that developed aspects of this conjecture in a variety of directions.
"But none has lifted the central theme of the book."
Gail Rebuck, chief executive of Random House, issued a statement about the case saying her company had published both books.
"We are genuinely saddened that two of the three authors of HBHG have chosen to bring litigation against us.
"Random House takes no pleasure in defending legal action that it believes is without merit and we are confident that we shall prevail."
Conan Chitman, a copyright specialist for Mishcon de Reya solicitors, said the case could have wide-ranging implications.
"This case, if it goes in favour of The Holy Blood and the Holy Grail, could open a floodgate of litigation for people who have had their ideas, as they see it, stolen by more successful people," he said.