Michael Baigent and Richard Leigh, authors of the 1982 nonfiction book "The Holy Blood and the Holy Grail," are suing publisher Random House, claiming that parts of their work formed the basis of Brown's 2003 novel, which has sold more than 40 million copies. The lawsuit, if successful, could delay the scheduled May opening of the Tom Hanks film based on Brown's novel.
The publisher's lawyer, John Baldwin, said the claim by Baigent and Leigh "relates to and seeks to monopolize ideas at such a high level of generality that they are not protected by copyright."
Brown, who is expected to testify next week, was in court for the second day of the trial. It was expected to recess later in the day until next Tuesday, providing time for the judge, Peter Smith, to read both books and related texts.
Baigent and Leigh's book claims Jesus married Mary Magdalene, that the couple had a child and that the bloodline survives. If they succeed in securing an injunction to bar the use of their material, they could hold up the May 19 release of "The Da Vinci Code" film, starring Tom Hanks and Ian McKellen.
Sony Pictures said it planned to release the film as scheduled.
Baldwin said many important themes of "The Holy Blood and the Holy Grail" did not feature in "The Da Vinci Code," particularly the idea that a secretive order called the Priory of Sion exists and seeks to restore descendants of Jesus Christ to the thrones of Europe.
Baldwin said this was a "massive point" in the earlier book but was not stressed by Brown.
Baigent, born in New Zealand, and Leigh, originally from the United States, are suing Random House, which also published their book.
The company denies the claim and chief executive Gail Rebuck said in a statement she believed the lawsuit was without merit.
The book's third writer, Henry Lincoln, is not involved in the case.
Jonathan Rayner James, a lawyer for the plaintiffs, said Monday that his clients were not attempting to "stultify creative endeavor" or to claim a monopoly on ideas or historical debate, but to prove Brown had "relied heavily" on the earlier work.
Baldwin said Brown developed most of the central ideas of his book independently.
"He found the ideas that he wanted to use in his novel before either he or his wife had looked at 'The Holy Blood and the Holy Grail,"' Baldwin said. "The story line was very well developed. He had the ideas in place, and decided to use them."
Baldwin said many of the ideas the claimants sought to protect were not original.
"They are ideas the claimants have taken from other sources," he said.
Brown's book also was the target of a previous lawsuit. In 2005, a U.S. judge in New York ruled that his book did not infringe on the copyrights of "Daughter of God," by Lewis Perdue. The judge also ruled out any copyright violations of Perdue's 1983 novel, "The Da Vinci Legacy."