Author Dan Brown has been accused of plagiarism in The Da Vinci Code just days after he was cleared in London's High Court of similar allegations.
Russian art historian Dr Mikhail Anikin, a Leonardo Da Vinci expert, wants Brown to apologise and hand over half the money made from his book within the next month.
If his demand is not met, Anikin claims he'll sue Brown in both Russia and the US for all his Da Vinci Code earnings.
Anikin says his 2000 book 'Leonardo Da Vinci: Theology in paint' argues that the Mona Lisa was an allegory for the Christian Church.
In 1998, Anikin alleges he discussed his ideas about the painting with some experts from Houston, Texas, one of whom asked if he could pass on the theory to Brown.
The Russian says he agreed so long as he was credited in any book, but never heard back. He also claims to have called his theory The Da Vinci Code.
Anikin says: "When I read the book, I was shocked at its poor quality and because it used my ideas. This book tells lies about the church which upset me morally."
Last Friday, Brown was found not to have breached copyright and plagiarised The Da Vinci Code from the 1982 book 'The holy blood and the holy grail' by Michael Baigent and Richard Leigh.
Director Ron Howard's movie adaptation of the best-seller, starring Tom Hanks and Audrey Tautou, is due out next month.