Patrick Janson-Smith, now a top literary agent, said he had warned at the time that Michael Baigent and Richard Leigh “didn’t have a leg to stand on” when they sued Random House, claiming The Da Vinci Code author Dan Brown had lifted from their own 1982 bestseller, The Holy Blood and the Holy Grail.

Mr Janson-Smith said he had published the paperback version of The Holy Blood and the Holy Grail, a book he had enjoyed and recognised would be commercially successful. When he became involved with The Da Vinci Code in 2001, he said he recognised the similarities between the two books and Dan Brown’s “joke” of using the name Sir Leigh Teabing as one of his characters - an anagram of Leigh and Baigent’s name.

“I assumed that because of the Sir Leigh Teabing character, it was likely that Mr Brown had at some point been in correspondence with the authors or, if not, that it was simply his nod of respect to them.

“I never thought to raise any concerns about plagiarism,” said Janson-Smith. He went on to say The Holy Blood and the Holy Grail “purports to be non-fiction”, while The Da Vinci Code is a thriller.

“I thought the latter was a romping piece of good fiction. Like any thriller, no doubt it took ideas from any number of sources.”

In 2004 Janson-Smith was contacted by Ann Evans, the literary agent acting for Baigent and Leigh, who told him that her clients were thinking of suing Dan Brown.

“I think I dismissed the idea of a court action, questioning aloud on what grounds they thought they stood to mount such an action. I may have said something like ‘Oh my God! Why the fuss? They wouldn’t stand a prayer,’ or something similar.”

When there was mention that The Da Vinci Code might be made into a film, Ann Evans rang him again saying that her authors were still minded to take action. She told him they were anxious that a film of The Da Vinci Code would jeopardise the chances of a film of The Holy Blood and the Holy Grail.

“I remember telling her that I thought the film rights to The Holy Blood and the Holy Grail had been sold a long time ago and that nothing had come of it; that it was ancient history.

“I’m pretty certain that I reiterated the fact that I didn’t think Baigent and Leigh had a leg to stand on and that they were in danger of making fools of themselves,” said Janson-Smith

Mr Brown now faces his third day of questioning over the similarities of certain passages in his novel with themes from The Holy Blood and the Holy Grail. The authors are suing Random House - publishers of both books - claiming that Mr Brown’s novel breached their copyright.

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