The author of a thriller that has sold more than 12 million copies is to be sued for plagiarising two books published more than 20 years ago.

Dan Brown, whose The Da Vinci Code is claimed by the publisher to have become the best-selling hardback adult novel ever, is expected to face legal action by the authors of a 1982 non-fiction bestseller and a 1983 novel.

Michael Baigent, Richard Leigh and Henry Lincoln, whose The Holy Blood and The Holy Grail was condemned by the Catholic Church but continues to draw readers and disciples, are said to be preparing to sue him for alleged breach of copyright of ideas and research.

Another author, Lewis Perdue, is threatening to sue Brown for alleged plagiarism, claiming that he borrowed heavily from Perdue's novel The Da Vinci Legacy.

The authors of The Holy Blood and The Holy Grail claimed to have found evidence that the Priory of Sion - a secret society founded in the late 11th century linked to the Knights Templar, and whose grand masters supposedly included Leonardo da Vinci, Victor Hugo and Isaac Newton - guarded documents that challenged orthodox Christian tenets and history.

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The writers claimed Jesus married Mary Magdalene and that their offspring fled the Holy Land and found refuge in a Jewish community in the south of France, thus preserving their lineage.