Authors Michael Baigent and Richard Leigh are suing "Da Vinci Code" publisher Random House for copyright infringement, claiming Brown "appropriated the architecture" of their 1982 nonfiction book "The Holy Blood and the Holy Grail."
Answering a question by the accusers' lawyer, reports CBS News correspondent Larry Miller, author Dan Brown said much of the research for the Da Vinci Code had been done by his wife who was "deeply passionate about the sacred feminine."
He admitted a debt to the non-fiction book, but said his research came from a whole raft of sources. History he said, can not be copyrighted.
Both books explore theories — dismissed by theologians but embraced by millions of readers — that Jesus married Mary Magdalene, the couple had a child and the bloodline survives.
If the writers succeed in securing an injunction to bar the use of their material, they could hold up the scheduled May 19 release of "The Da Vinci Code" film starring Tom Hanks and Ian McKellen.
Brown's lawyers released a previous witness statement in which the author said he was shocked by the allegations.
"Baigent and Leigh are only two of a number of authors who have written about the bloodline story, and yet I went out of my way to mention them for being the one who brought the story to mainstream attention," Brown said in the statement.
"I have been shocked at their reaction; furthermore I do not really understand it."
The plaintiffs' allegations contain "numerous sweeping statements which seem to me to be completely fanciful," Brown said.
Random House lawyers argue that the ideas in dispute are so general they are not protected by copyright. They also say many of the ideas in "The Holy Blood and the Holy Grail" do not feature in Brown's novel, which follows fictional professor Robert Langdon as he investigates the murder of an elderly member of an ancient society that guards dark secrets about the story of Jesus and the quest for the Holy Grail.
Brown, 41, has traveled from his home in the New Hampshire to sit in courtroom 61 of London's High Court as lawyers dissect the books' accounts of the medieval Knights Templar, the secretive Priory of Sion and the Merovingian royal line allegedly descended from Jesus.
Lawyers, witnesses and judge Peter Smith have pored over well-thumbed paperback copies of the two books and consulted related works with titles like "The Templar Revelation" and "The Woman with the Alabaster Jar."
During more than three days of cross-examination last week by Random House lawyer John Baldwin, Baigent conceded that several key points in the claimants' case were wrong — including the claim that "The Da Vinci Code" contained all the same historical conjecture as the earlier book.
"I would concede that 'all' is far too strong," Baigent said. "I would say 'most.'"
Brown's fast-moving theological thriller has sold more than 40 million copies since it was published three years ago this week. "The Holy Blood and the Holy Grail" — a more modest best-seller on its release — has seen a surge in sales since the trial began, going from about 350 copies sold a week in Britain to 3,000, according trade publication The Bookseller.
The third author of "The Holy Blood and the Holy Grail," Henry Lincoln, is not involved in the case. A lawyer for the plaintiffs, Paul Sutton, refused to say why he was not participating.
Lincoln, who is in his 70s and reportedly in poor health, could not be reached for comment.