There are 25 of them in all, of different shapes and sizes, complete with testicles. They hang from the limbs of the tree like leaves fluttering in the breeze. The mural dates from the 13th century and is still visible on a wall in the Italian town of Massa Marittima where it was discovered during renovation work four years ago.

Until now, it was assumed the phallus tree was a fertility symbol because it stands by a fountain - the town's main source of water in medieval times. But, according to a British-based expert, it is actually a unique piece of political propaganda, commissioned by one Tuscan faction to sully the reputation of another. "It's a very clear political poster," said George Ferzoco, director of the Centre for Tuscan Studies at the University of Leicester in central England as he unveiled his theory about the picture for the first time.

"It's a message from the Guelphs, telling people that if the Ghibellines are allowed power they will bring with them heresy, sexual perversion, civic strife and witchcraft." The Guelphs and Ghibellines were two factions who fought for power in Tuscany and northern Italy for decades during the Middle Ages.

Perhaps the most famous victim of their feuds was the poet Dante, a Guelph expelled from his native Florence in 1302 after a rival Guelph group took power. At the time the mural was painted, the Guelphs controlled Massa Marittima, a small town in northwestern Tuscany. "They presented themselves as the clean living upstanding party in Tuscan politics and it was traditional for them, in launching their attacks on the Ghibellines, to label them as heretics," Ferzoco told Reuters.

"Heretics, according to people in the Middle Ages, practised sodomy. Hence the phallus tree." It may seem a cryptic message to the modern mind but Ferzoco says it would have been obvious to the average medieval Tuscan.

"They would have got the message instantly," he said. "They considered things we consider obscene to be perfectly normal, and using a phallus as part of a political message would not have been an issue." Ferzoco says the robed women standing under the phallus tree are witches - another curse the Guelphs claimed the Ghibellines would bring to the town. One of the women appears to be reaching up and placing something in the lower branches of the tree with a stick.

"There was a well-known story in Tuscan folklore about witches removing mens' penises and placing them in bird nests in trees, where they would then multiply and take on a life of their own," Ferzoco said. He says the picture draws on that story and is one of the earliest known depictions of witches in Western art.

- Reuters