An author who claims "The Da Vinci Code" copied ideas from his work insisted in a British court Wednesday that there are major similarities between his nonfiction book and Dan Brown's crypto-religious thriller -- but conceded there are also substantial differences.

"We were writing historical conjecture, and Mr. Brown was writing a novel," said Michael Baigent, co-author of "The Holy Blood and the Holy Grail."

"One would expect their perspectives to be marginally different, if not substantially different."

Nonetheless, he said, the similarities were "fairly specific."

Baigent and co-author Richard Leigh are suing "Da Vinci Code" publisher Random House at Britain's High Court for infringing the copyright of their 1982 book. They claim Brown's blockbuster "appropriated the architecture" of their work, which explores theories 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.

Random House lawyers argue that the ideas in dispute are so general they are not protected by copyright.

A lawyer for the publisher, John Baldwin, also said many of the ideas in "The Holy Blood and the Holy Grail" did not feature in Brown's book, a mixture of code-breaking, art history, religion and mystical lore that has sold more than 40 million copies since it was published in 2003.

"Your statement that Mr. Brown reached all the same historical conjecture you did is fairly misleading," Baldwin told Baigent. "You are being unfair and inaccurate."

Baigent insisted that Brown "used the results of our historical conjecture."

"'The Da Vinci Code' uses the tips of the icebergs that were produced by the research that we did," Baigent said.

New Zealand-born Baigent was appearing for a second day of tense exchanges with Baldwin. At one point, he acknowledged using "infelicitous phraseology" in his witness statement.

"Is infelicitous your long word for being wrong?" said Baldwin.

In a day of testimony that sometimes evoked a university seminar, lawyers, judge and witness pored over well-thumbed copies of "The Da Vinci Code," seeking parallels and differences with "The Holy Blood and the Holy Grail."

Baldwin read passages from the novel out loud to stress the differences from the earlier book, which proposed that a secret order called the Priory of Sion existed to preserve the bloodline of Jesus Christ and Mary Magdalene.

"There's no mention of the Priory's oath to keep its true nature hidden in `The Holy Blood and the Holy Grail,' is there?" Baldwin asked.

Baigent replied: "No. I concede that."

"There's no mention of the Priory protecting the tomb of Mary Magdalene, is there?" Baldwin asked.

"Not explicitly, no," said Baigent.

Judge Peter Smith told Baigent that it appeared some theories put forward in "The Da Vinci Code" were "exactly the opposite" of passages in the earlier book.

Smith said Dan Brown's book used some of the same material "and comes up with an entirely different plot. Are you saying they can't do that?"

"No I'm not, my lord," Baigent said.

Leigh is scheduled to take the stand Thursday, with Brown to follow, likely on Friday.

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.