Richard Leigh and Michael Baigent are suing Random House for lifting "the whole architecture" of the research that went into their 1982 non-fiction book "The Holy Blood, and the Holy Grail".
Lawyers on both sides of the case met on Thursday to thrash out technical details, and said a trial date had been set for February 27.
They would not comment on how the trial might affect sales of the hugely successful novel or the distribution of a major Hollywood adaptation which Sony Pictures plans to release in May next year.
Random House said a "substantial" part of the claim by Baigent and Leigh had been dropped as a result of Thursday's discussions, and added in a statement:
"Random House is delighted with this result, which reinforces its long-held contention that this is a claim without merit."
A spokeswoman for Leigh said he still intended to pursue his claim against the publishers of Brown's book, which has 36 million copies in print world-wide and has upset Catholics for suggesting Jesus married Mary Magdalene and had a child by her.
The same theory is put forward in The Holy Blood, and the Holy Grail.
Commentators have pointed out that a major character in Dan Brown's book, Sir Leigh Teabing, has a name that is an anagram of Leigh and Baigent. A third author of the 1982 book, Henry Lincoln, has decided to stay out of the action.
Ironically, a special hardback, illustrated version of their book, called Holy Blood, Holy Grail has just been reissued by none other than Random House.
In August, Brown won a court ruling against another writer, Lewis Perdue, who claimed The Da Vinci Code copied elements of two of his novels, "Daughter of God" and "The Da Vinci Legacy".
Perdue had sought $150 million in damages and asked the court to block distribution of the book and the movie adaptation, which features Tom Hanks alongside French actress Audrey Tautou.