Authors Michael Baigent and Richard Leigh sued Random House, claiming Brown's best-selling novel "appropriated the architecture" of their 1982 nonfiction book, The Holy Blood and the Holy Grail.
"It would be quite wrong if fictional writers were to have their writings pored over in the way DVC (Da Vinci Code) has been pored over in this case by authors of pretend historical books to make an allegation of infringement of copyright," Judge Peter Smith said in his ruling.
Both books explore theories that Jesus married Mary Magdalene, the couple had a child and the bloodline survives. The idea had been dismissed by most historians and theologians.
The popular novel the Da Vinci Code sold more than 40 million copies since its release in March 2003. Sales of the book are expected to increase with the planned May 19 release of a film version starring Tom Hanks.
Copyright issues
"Cases like this hopefully will clarify what copyright is all about," said Allan Adler, vice president for legal and government affairs at the Association of American Publishers.
"Copyright doesn't protect ideas and copyright doesn't protect facts. That's why we have genres, fiction and nonfiction, and a number of people can write novels based on the same idea and still have freedom of expression."
Brown was called to testify in the London court for a few days and then returned to his home in the US state of New Hampshire.
He said in a statement that a novelist must "be free to draw appropriately from historical works without fear that he'll be sued and forced to stand in a courtroom that call into question his very integrity as a person."
It’s an expensive embarrassment for Baigent and Leigh who spent two years on the lawsuit and nearly US$3.5 million (AU$4.8 million), between their own legal expenses, and Random House's fees.
However the pair can still apply to higher courts but Leigh has said that as yet no decision on an appeal has been made. The authors have been ordered to pay an interim payment of US$600,000 (A$823,215) by May 5.
Random House may end up the biggest winner. Its legal costs are covered and, as publisher of The Da Vinci Code and The Holy Blood and the Holy Grail, it benefits from increasing sales for both books.