The Da Vinci Code author Dan Brown has denied claims that he lied about when exactly he first saw a copy of The Holy Blood and The Holy Grail.
Random House published both books at the centre of the dispute
The writer, who was giving evidence at London's High Court, dismissed claims that he read the book while his novel's synopsis was being prepared.
Michael Baigent and Richard Leigh are suing publishers Random House, claiming Mr Brown stole ideas from their book.
Mr Brown, 41, is expected to remain in the witness stand until Wednesday.
Mr Baigent and Mr Leigh's lawyer Jonathan Rayner James said to Mr Brown: "You acquired a copy of The Holy Blood and Holy Grail before you accept in your evidence you did."
Mr Brown said the book did not appear among the seven works cited in the synopsis for The Da Vinci Code.
"That is the clear piece of evidence to me that Holy Blood, Holy Grail was not around when I wrote the synopsis," he said.
The 41-year-old added that it would have been in his interest to include it in his bibliography as it would have impressed his publishers.
Mr Rayner James said it was listed as "essential reading" in another book he wrote before the synopsis for The Da Vinci Code.
Mr Brown replied: "I had everything I needed for that synopsis. I'm in a synopsis phase. I'm looking at the big picture, not the details."
Mr Brown has maintained that neither he nor his wife and assistant Blythe Brown used The Holy Blood and The Holy Grail while his best-selling book was being prepared.
New Zealand author Michael Baigent is one of the co-claimants
But he has admitted to using the work while The Da Vinci Code was being written, but added it was used as one of several sources and did not use its central themes.
"I have never been shy about saying Holy Blood, Holy Grail is part of this," Mr Brown said.
"The whole Teabing section of the book - those are the sorts of snippets of information that Holy Blood, Holy Grail is very good on," he added.
Leigh Teabing, a character in the novel, is an anagram of the names of the two claimants in the case, and their book is directly referred to in the narrative.
The case is expected to last until next Monday, while a judgment could take several weeks to be reached.