Michael Baigent and Richard Leigh, authors of "The Holy Blood And The Holy Grail," now face legal bills of about 3 million pounds (US$6 million; EUR4.4 million) after losing their appeal against publisher Random House Inc.
Baigent and Leigh had argued that Brown stole significant elements from their book. Both books are based on a theory that Jesus and Mary Magdalene married and had a child, and that the bloodline continues to this day.
Their lawyers said Baigent and Leigh had "expended a vast amount of skill and labor" on their book, published in 1982. "That skill and labor is protectable."
Brown testified for several days during the High Court hearing last year.
The claimants' lawyer, Jonathan Rayner James, said that although the suit had been against the publisher rather than the author, Brown was really the one being put on trial.
During a hearing earlier this year, Rayner James said issues remained about the role of Brown's wife, Blythe, who did much of the research for the blockbuster novel. She did not testify at the High Court hearing. Brown said he wanted to protect his wife from publicity.
In April, Justice Peter Smith ruled that Random House, publisher of "The Da Vinci Code," had not breached the copyright. Smith said the claim was based on a "selective number of facts and ideas artificially taken out of (the book) for the purpose of the litigation."
"The Da Vinci Code" has sold more than 40 million copies since its release in March 2003. A film version starring Tom Hanks was released last year.
Lord Justice Bernard Rix said Brown had not disguised his use of the work of Baigent and Leigh.
The character of Leigh Teabing is an anagram of Leigh and Baigent, Rix noted, and at one point Teabing refers to "The Holy Blood and the Holy Grail" as "perhaps the best-known tome" on the subject.
"That is not the mark of an author who thought that he was making illegitimate use of the fruits of someone else's literary labors, but of one who intended to acknowledge a debt of ideas, which he has gone on to express in his own way and for his own purposes," Rix wrote in his opinion.