Both books involved in the case are published by Random House

Michael Baigent and Richard Leigh are suing Random House over the copyright of their 1982 book, The Holy Blood and the Holy Grail.

They claim author Dan Brown pitched his book to publishers in a synopsis using the same key themes from their book.

But Mr Baigent admitted the themes were not made in the same order.

Art history

He conceded the point whilst being cross-examined by Random House lawyer John Baldwin at the High Court in London on Thursday.

Mr Baigent and Mr Leigh claim Mr Brown's best-seller "appropriated the architecture" of their work, which explores theories that Jesus married Mary Magdalene, the couple had a child and the bloodline survives.

Mr Baldwin also argued that the authors were being unfair to Mr Brown, who wrote the best-selling thriller.

"Your castigatory assertion... that Mr Brown reached all the same historical conjecture as you is untruthful," Mr Baldwin said.

Mr Baigent replied: "I would concede that 'all' is far too strong. I would say 'most'... We over-egged [exaggerated] that one."

Mr Baldwin also argued that many of the ideas in The Holy Blood and the Holy Grail do not feature in Mr Brown's book, a mixture of code-breaking, art history and religion.

While Mr Baigent insisted that "our basic conclusion was the fundamental theme of The Da Vinci Code", he also acknowledged some of the "details and incidents" in the earlier book were not in The Da Vinci Code.

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

The third author of The Holy Blood and the Holy Grail, Henry Lincoln, is not involved in the case.