A Russian art historian has accused The Da Vinci Code author Dan Brown of plagiarism, just days after a British court rejected a similar claim.
The Da Vinci Code has sold more than 40m copies worldwide
Mikhail Anikin, from St Petersburg, said he would sue Brown if he did not receive an apology and compensation.
He claims Brown stole his idea that Leonardo Da Vinci was also a theologian and his Mona Lisa portrait was an allegory for the Christian Church.
The Da Vinci Code was published in 2003 and is a global blockbuster.
Mr Anikin, a Da Vinci expert at the Hermitage Museum, said he had shared his ideas with colleagues at a museum in Houston, Texas, in 1998.
He said one of the them had asked if he could pass on the ideas to a friend who wrote detective novels.
"I gave permission, but asked that this author indicate in his book that the idea had been mine," Mr Anikin told Agence France Presse.
On Friday, the High Court rejected a claim that Brown had breached the copyright of the 1982 book The Holy Blood and the Holy Grail.
Both books explore the theory that Jesus and Mary Magdalene had a child and the bloodline survives to this day.
The Da Vinci Code is still in the UK top 10 book sales chart, and a screen adaptation starring Tom Hanks is due for release in May.