Patrick Janson-Smith also warned Michael Baigent and Richard Leigh "were in danger of making fools of themselves," the BBC reported Wednesday.
The authors of "The Holy Blood and the Holy Grail" are suing their own publisher, Random House, claiming Brown's book -- also a Random House product -- stole ideas they had written in 1982.
Brown has rejected the plagiarism charge as absurd, both in and out of court.
Janson Smith -- who handled both books -- told the High Court he saw some similarities between the two books, but "I never thought there was a copying issue to deal with."
He also implied the claimants were motivated to file suit because of director Ron Howard's upcoming Hollywood version of "The Da Vinci Code," not Brown's alleged thievery.
"They were most especially concerned that a film of 'The Da Vinci Code' would jeopardize the chances of a film of 'The Holy Blood and the Holy Grail' ever being made," Janson-Smith testified.