Making his closing submission at the end of a three-week trial, attorney Jonathan Rayner James also asked why Brown's wife, Blythe — who did a large chunk of the research for the novel — wasn't called as a witness in the copyright-infringement case.
Michael Baigent and Richard Leigh are suing "Da Vinci Code" publisher Random House, claiming Brown's book "appropriated the architecture" of their 1982 nonfiction book "The Holy Blood and the Holy Grail." Both books explore theories — dismissed by theologians — that Jesus married Mary Magdalene, the couple had a child and the bloodline survives.
The case was being put in the hands of Justice Peter Smith when closing arguments concluded. His decision wasn't expected for two weeks or so.
In a written statement handed to the court Monday, Rayner James said Brown had copied from "The Holy Blood and the Holy Grail," but acknowledged he may have done so "unwittingly because of the research materials supplied by Blythe Brown."
"His evidence should be approached with deep suspicion," the lawyer said.
"He had almost no recollection of matters that related to timing. He would struggle to recall a year, was rarely able to recall a month. His general attitude in cross-examination was uncooperative," Rayner James said, referring to Brown's testimony during three days on the stand last week.
Brown, who flew from his home in New Hampshire to give evidence, was not in court Monday.
Rayner James said evidence from Blythe Brown would have been of "fundamental importance to this case." He claimed she would have been able to confirm the extent to which "The Da Vinci Code" relied on Baigent's and Leigh's work.
Dan Brown knew "little about what she did," Rayner James said.
"It remains the position that only she knows the extent of her involvement in the research and creation" of "The Da Vinci Code," he added.