Leonardo da Vinci left the Mona Lisa unfinished because he gravely injured his arm while fainting, a new study argues.

The cause of the renaissance artist’s disability has been debated by art historians for centuries, and in recent years partial paralysis as a result of a stroke has emerged as the dominant theory.

Proponents have pointed to da Vinic’s vegetarianism as a clue, arguing that the high-dairy diet he is assumed to have eaten would have made a stroke more likely.

However, two senior Italian doctors now claim to have solved the mystery, having studied a drawing of da Vinci by an obscure Lombard artist.

The blood-red chalk picture by Giovan Ambrogio Finio depicts an elderly da Vinci with his lower right arm at right-angles to his body, swaddled in folds of his clothes as if in a sling.

His thumb, first and second figures are extended, with his fourth and fifth fingers are contracted.

Writing in the Journal of the Royal Society of Medicine, Dr Davide Lazzeri, a plastic surgeon, and Dr Carlo Rossi, a neurologist, argue that if da Vinci had indeed suffered a stroke, it is far more likely his entire fist would have been clenched.

They also point out there is no evidence the old master suffered any facial or neurological impairment, which is common following a stroke.