Mikhail Anikin, a Leonardo da Vinci expert in the Hermitage museum’s Western European art department, said he would give Mr Brown one month to apologise and give up half his revenues from the book or he would take him to court in Russia and the US to seek all his earnings from the novel.

“When I read the book, I was shocked at its poor quality and because it used my ideas,” Dr Anikin said. “This book tells lies about the Church which upset me morally.”

Dr Anikin said he had written a book called Leonardo da Vinci: Theology In Paint in 2000, in which he argued that the Mona Lisa was an allegory for the Christian Church.

Two years ealier, he said, he had shared his views on the painting with some visiting specialists from the Menil Collection of Houston, Texas, who helped to organise an exhibition at the Hermitage. One, he said, had asked if could pass on the ideas to Brown, describing him as “a friend who wrote detective novels”.

Dr Anikin said that he agreed and even gave his theory the name, The Da Vinci Code, but insisted that he should be credited in any book.He never heard back, he said.

The Da Vinci Code was published in 2003 and soon became a global blockbuster.

On Friday, the High Court in London rejected a claim that Brown had plagiarised the 1982 book Holy Blood, Holy Grail by Michael Baigent, Henry Lincoln and Richard Leigh. Dr Anikin said he had heard about that result, but was nonetheless confident of his case. He said he had not spoken out earlier because he had an agreement with a Russian magazine giving it exclusive rights to his story.

Clare Harrington, a spokeswoman for Random House, which published The Da Vinci Code, said the company did not want to comment on “unsubstantiated threats”.

“We would suggest that anyone making claims with regard to The Da Vinci Code reads the judgement following the trial at the High Court,” she added.

There was no response from Brown.

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