Leigh met his frequent co-author Michael Baigent while living in the United Kingdom in the 1970s. They subsequently struck a friendship with the writer and British television scriptwriter Henry Lincoln in 1975 and between them developed a conspiracy theory involving the Knights Templar and the alleged mystery of Rennes-le-Château, proposing the existence of a secret that Jesus had not died on the Cross, but had married Mary Magdalene and fathered descendants who continued to exert an influence on European history. This hypothesis was later put forward in their 1982 book, The Holy Blood and the Holy Grail. The Holy Blood and the Holy Grail achieved enormous commercial success and has been described as "one of the most controversial books of the 1980s". It popularised the idea that the true object of the quest for the Holy Grail was to find secret descendants of Jesus and Mary Magdalene. This bloodline is stated to have later married into a Frankish royal dynasty, the Merovingians, and to be championed and protected by a secret society known as the Priory of Sion. These notions were later used as a basis for Dan Brown's international best-selling novel The Da Vinci Code. The day after publication, the authors had a public clash on BBC television with the Bishop of Birmingham and Marina Warner. The book rapidly climbed the best-seller charts, and the authors published a sequel, The Messianic Legacy, in 1986. The book has been described as "a work thoroughly debunked by scholars and critics alike". Arthurian scholar Richard Barber has commented, "It would take a book as long as the original to refute and dissect The Holy Blood and the Holy Grail point by point: it is essentially a text which proceeds by innuendo, not by refutable scholarly debate".
Other works
In 1991 Leigh published The Dead Sea Scrolls Deception, co-authored with Baigent. The book follows the controversial theories of Robert Eisenman regarding the Dead Sea Scrolls. Two books of Leigh's fictional works have been published: Erceldoune & Other Stories, and Grey Magic.
Some of the ideas presented in Baigent's earlier book The Holy Blood and the Holy Grail, were incorporated in the best-selling American novel The Da Vinci Code, by Dan Brown. In March 2006, Baigent and Leigh filed suit in a British court against Brown's publisher, Random House, claiming copyright infringement. On 7 April 2006High Court judgePeter Smith rejected the claim. On 28 March 2007, Baigent and Leigh lost their appeal, and were faced with legal bills of about £3m.
