Two historians who lost a plagiarism case against "The Da Vinci Code" author Dan Brown launched an appeal on Tuesday to have the verdict overturned.

Michael Baigent and Richard Leigh, who wrote "The Holy Blood and the Holy Grail" which they say Brown copied, were at London's High Court to hear the opening of the appeal.

Their lawyer, Jonathan Rayner James, will argue that the original judge was wrong to dismiss the idea of a "central theme" in the historians' research which he says was used extensively in six chapters of "The Da Vinci Code."

"The ... judge 'rejected' the central theme because, inter alia, it was not the substantial part of HBHG (The Holy Blood and the Holy Grail)," he said in a printed summary of his argument to be presented in court.

"This approach is incorrect; it does not have to be the substantial part of HBHG ...

"Was there enough of an expenditure of skill and labor to warrant copyright protection? The appellants submit that there was."

Brown, who was called as a witness during the original case last year, is not expected to be present for the appeal.

Judge Peter Smith ruled in April that the central themes were too general or abstract to be protected by copyright.

Brown said at the time that novelists must be allowed to draw from historical works without fear of being sued.

"The Da Vinci Code" is one of the most successful novels of all time, selling more than 40 million copies worldwide and being turned into a major Hollywood movie. It is published in Britain by Random House, as is "The Holy Blood."

The appeal is likely to focus on legal argument, and lack the original case's colorful and often heated debate about the Merovingian monarchy, the knights Templar and Jesus' bloodline.

Both "The Da Vinci Code" and "The Holy Blood and the Holy Grail" raise the possibility that Jesus had a child by Mary Magdalene, that she fled to France after the Crucifixion and that Christ's bloodline survives to this day.

They also associate Magdalene with the Holy Grail.

As in the case last year, the appeal will also focus on Brown's wife Blythe, who emerged as a key contributor to "The Da Vinci Code" through research and ideas.

"In this case, Mrs. Brown knew that she was exclusively using HBHG for that subject matter comprising the Langdon/Teabing lectures," Rayner James said, referring to the six chapters around which the case centers.

"She took a 'short cut' through the research and investigation and simply lifted the material from HBHG."

The appeal is expected to wind up on Friday. The judgment is likely to be delivered in written form at a later date.