Michael Baigent, one of two historians suing Brown’s British publisher Random House for copyright infringement, took the witness stand at London’s High Court in a closely watched case that could change the way writers research their work.

He is making the claim with Richard Leigh, a co-author of the 1982 book "The Holy Blood, and the Holy Grail," which, like The Da Vinci Code, explores the idea that Jesus married Mary Magdalene and had a child by her.

The historians argue that Brown copied much of the 15 central points they say make up "The Holy Blood,’’ which is also published by Random House and sold around two million copies.

Baigent added that The Holy Blood book was the result of five years of research between 1976 and 1981.

During Baigent’s cross examination, the court was treated to quips from the judge and discussions on obscure historial theories including the Knights Templar, the Priory of Sion and the restoration of Jesus’ bloodline to the thrones of Europe.

Brown, 41, who has attended hearings since the case began, is expected to give evidence late this week or early next week.

Baigent was forced to concede that some of the 15 points of the central theme did not appear in The Da Vinci Code at all.

Justice Peter Smith also called the historians’ definition of the central theme a "moving feast," after elements were dropped from an original outline just before the case began.

But New Zealand-born Baigent countered that what was in The Da Vinci Code was the result of years of other people’s work.

"We dealt with the body of the iceberg," he said. "The tips exist in The Da Vinci Code. The tips could not have existed without the body of the iceberg."

With more than 36 million copies of Brown’s novel in circulation, a major Hollywood adaptation due for release in May and a potentially important precedent in copyright law being set, the stakes at London’s High Court are high.

However, Sony Pictures Entertainment, which is making the movie starring Tom Hanks and Audrey Tautou, appears unfazed.

"This lawsuit is not about the movie, and we are proceeding with our plans," said a spokesman for the Hollywood studio in Los Angeles.

Smith said the key point of the case may prove to be the sequence in which Brown read his sources.

In opening arguments last week, Random’s lawyers told the court Brown wrote a synopsis of The Da Vinci Code before he or his wife even looked at The Holy Blood.

As Brown’s side concedes, the name of one character in The Da Vinci Code, Sir Leigh Teabing, is an anagram of Leigh and Baigent, and Teabing refers to their work in the narrative. But it adds that Brown took his research from a number of sources.

Last August, Brown won a court ruling against another writer, Lewis Perdue, who claimed The Da Vinci Code copied elements of two of his novels.

The restoration of a Merovingian monarchy descended from Jesus, emperor Constantine’s conversion, the Knights Templar and the mysterious Priory of Sion might normally be reserved for rarefied academic debate.

But over the last two weeks, they and other arcane themes from ancient and medieval history or folklore have been thrust into the here and now in a gripping copyright case that enters its third, and possibly final week on Monday.

Dozens of reporters from around the world have followed the case’s every twist and turn, and joke that they smell a conspiracy theory worthy of The Da Vinci Code itself.

With both books likely to benefit from the publicity, could Random House be behind it? Not so, says the publisher.

"As publisher of both ‘The Da Vinci Code’ and ‘The Holy Blood and The Holy Grail,’ we are genuinely saddened that two of the three authors of HBHG have chosen to bring this litigation against us," it said.

For another thing, it would take a lot of book sales to cover costs in the case which experts estimate at over 1 million pounds (.75 million).