Michael Baigent had said 15 points central to the plot of Mr Brown’s novel were from a 1982 non-fiction book Mr Baigent wrote with two other authors.
Mr Baigent and Mr Leigh claim Mr Brown “relied heavily” on their work, The Holy Blood and the Holy Grail. The book’s third author, Henry Lincoln, is not involved in the case.
Random House, which published both books, has denied the allegation.
As the case resumed at the High Court in London, however, Mr Baigent said his language had been “infelicitous”.
He and co-author Richard Leigh are suing UK publisher Random House.
They have taken the publisher to court for copyright infringement. Mr Brown is expected to take the stand later this week.