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Richard Leigh, a writer of alternative history who unsuccessfully sued for plagiarism over themes in Dan Brown's blockbuster novel "The Da Vinci Code," has died, his agent said Friday. He was 64.

U.S.-born Leigh, who had lived in Britain for three decades, died in London on Nov. 21 of causes related to a heart condition, the Jonathan Clowes Agency said.

Leigh was co-author of "The Holy Blood and The Holy Grail," a work of speculative nonfiction that claimed Jesus Christ fathered a child with Mary Magdalene and that the bloodline continues to this day.

A best-seller on its release in 1982, the book gained new readers after Brown's thriller, which explores similar themes and has sold more than 40 million copies, was released in 2003.

Leigh and co-author Michael Baigent sued Brown's publisher Random House, claiming "The Da Vinci Code" "appropriated the architecture" of their book. A third "Holy Blood" author, Henry Lincoln, did not join the lawsuit.

The authors made a striking pair during the lengthy High Court hearing — Baigent professorial in a suit and tidy gray hair, Leigh sporting a leather jacket and thick sideburns.

In April 2006, High Court judge Peter Smith threw out the claim, saying the ideas in question were too general to be protected by copyright.

The prominent court case sent "Holy Blood" back up the best-seller lists, but Baigent and Leigh were left with a bill estimated at almost 3 million pounds (US$6.2 million; euro4.2 million) after the judge ordered them to pay 85 percent of Random House's legal costs.

"We lost on the letter of the law. I think we won on the spirit of the law; to that extent we feel vindicated," Leigh said after the verdict.

That was optimistic assessment. The judge harshly criticized the claimants, calling Baigent "a poor witness" and saying, "I am not sure what Mr. Leigh thought was the purpose of his evidence."

An attempt to appeal the ruling was rejected earlier this year.

Leigh, born in New Jersey in 1943 to a British father and Austrian mother, attended Tufts University in Boston, the University of Chicago and the State University of New York at Stony Brook. He worked as a university lecturer in the United States and Canada before settling in Britain.

In the mid-1970s he met Lincoln and Baigent, and the trio discovered a shared interest in the Medieval order known as the Knights Templar. They developed a thesis linking the knights with the Merovingian dynasty allegedly descended from Jesus.

Baigent and Leigh collaborated on several other books, including "Holy Blood" sequel "The Messianic Legacy"; "The Dead Sea Scrolls Deception," which alleged a Roman Catholic conspiracy to cover up the scrolls; and "Secret Germany," about a plot to kill Adolf Hitler.

Leigh's first love was fiction, and he published a book of stories, "Erceldoune and other Stories," and the semi-autobiographical novel "Grey Magic."
He never married. A funeral was held Wednesday.