New Zealander Michael Baigent and fellow historian Richard Leigh have lost the latest round of an epic legal battle in which they claim that author Dan Brown lifted their ideas for his bestseller The Da Vinci Code.
Three of Britain's senior judges dismissed their appeal against an earlier High Court ruling rejecting their claims.
The decision leaves them facing estimated legal costs of $5.5 million.
It seems unlikely that the case will go any further as no application has been made for permission to appeal to the House of Lords.
In their action against the novel's British publishers Random House, Baigent and Leigh claimed that Brown copied significant parts of The Holy Blood and the Holy Grail which they wrote in 1982.
Both books raise the possibility that Mary Magdalene had a child by Jesus, that she fled to France after the Crucifixion and that Christ's bloodline survives to this day.
In the appeal, the historians' lawyers argued that the High Court judge, Mr Justice Smith, who controversially wrote a secret code of his own into his 71-page judgement, had misunderstood the law and the basis of their claims.
They said it was wrong to dismiss the idea of a "central theme" in the historians' research which he said was used extensively in six chapters of The Da Vinci Code.
Brown's wife Blythe, who emerged during the original case last year as a key researcher and inspiration for the author, knew she was relying exclusively on the historians' book for the chapters, the lawyers said.
But the Appeal Court judges backed Brown.
"In my judgement, the judge rightly held that the claimants have not established that a substantial part of The Holy Blood and the Holy Grail has been copied," said Lord Justice John Mummery in his ruling.
Random House said it was glad "common sense and justice" had prevailed.
"We believe that the case should never have come to court in the first place, and regret that even more time and money was spent trying to appeal the original judgement," the publisher said in a statement.
"Misguided claims like the one that we have faced, and the appeal, are not good for authors and not good for publishers."
However Baigent and Leigh said they had not complained for the sake of it.
"We feel that today is an ominous one for those who wish to research a book on their own and come up with their own theories," they said in a joint statement.
"It is a carte blanche for those who would rather not bother but simply take another author's ideas and adapt them."
The Da Vinci Code is one of the most successful novels ever, selling more than 40 million copies worldwide, and has been turned into a Hollywood movie.