A British portrait of Napoleon Bonaparte has given new life to a theory that the French emperor’s tomb in Paris contains the remains of another man.

Bruno Roy-Henry, a historian and prominent Napoleonic conspiracy theorist, said that an 1815 painting of Napoleon on his way to exile in Saint Helena shows a scar on the left side of his face. No such scar exists on any French portrait or the emperor’s official death mask, which is on display at the Army Museum at Invalides, near his tomb on the Left Bank.

The painting of Napoleon on HMS Bellerophon by Charles Locke Eastlake was the only portrait of the emperor painted from life by a British artist. The work hangs in the National Maritime Museum, in Greenwich, London. Mr Roy-Henry said that an identical scar appears on another death-mask, which was in the possession of the Royal United Services Institute and sold by Sotheby’s to an American buyer in 2004. The buyer remains anonymous.

The authenticity of Napoleon’s remains has been doubted by conspiracy buffs almost since they were exhumed by the British in Saint Helena in 1840, 19 years after his death at the age of 52. Suspicions have grown in the past five years since the French authorities refused to allow DNA testing on the remains in Napoleon’s tomb.

Mr Roy-Henry and his colleagues believe that the British spirited away the original Boney and replaced him with Jean-Baptiste Cipriani, his maître-d’hotel on Saint Helena.

The features on the museum death mask have long been deemed too young for the corpulent exile who officially died of cancer.

Mr Roy-Henry acknowledged yesterday that there were doubts about the British death mask, which had passed through the hands of a well-known swindler before being given to the London Institute in 1947 by Charles Alder, a British millionaire.

“But the scar on the English picture is a powerful clue to its authenticity,” Mr Roy-Henry told The Times. “The English mask thus shows the true face of the emperor but the [Paris] Army Museum refuses to accept reality. It would be better if the false mask in the Invalides were removed quickly from public view.”

The Army Museum responded to the claims, saying that it had no concern about its mask. It said that no credence should be given to “disbelieving theorists who use the month of August to circulate old theses about the death of Napoleon”.

The other thesis, revived in recurrent studies and books, is that Napoleon did not die naturally. He is said to have succumbed to arsenic or lead administered by his British captors or by a jealous husband, or inhaled from wallpaper or plumbing.

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