The sword of St Galgano, said to have been plunged into a rock by a medieval Tuscan knight, has been authenticated, bolstering Italy's version of the Excalibur legend.

Galgano Guidotti, a noble from Chiusdano, near Siena, allegedly split the stone with his sword in 1180 after renouncing war to become a hermit. For centuries the sword was assumed to be a fake. but research revealed last week has dated its metal to the twelfth century.

Only the hilt, wooden grip and a few inches of the 3ft blade poke from the hill, which still draws pilgrims and tourists to the ruins of the chapel built around it.

'Dating metal is a very difficult task, but we can say that the composition of the metal and the style are compatible with the era of the legend,' said Luigi Garlaschelli, of the University of Pavia. 'We have succeeded in refuting those who maintain that it is a recent fake.'

Ground-penetrating radar analysis revealed that beneath the sword there is a cavity, 2m by 1m, which is thought to be a burial recess, possibly containing the knight's body. 'To know more we have to excavate,' said Garlaschelli, whose findings have been published in Focus magazine.

Carbon-dating confirmed that two mummified hands in the same chapel at Montesiepi were also from the twelfth century. Legend has it that anyone who tried to remove the sword had their arms ripped out.

In English legend the sword Excalibur is pulled from a stone by the future King Arthur, heralding his glory. In Galgano's case the miracle signified humility and holiness.

The son of an illiterate feudal lord, Galgano had a reputation for arrogance and selfishness. After a vision of the Archangel Michael, however, he retired to a cave to become a hermit. Lured out by his family he was thrown by his horse while passing Montesiepi, a hill near Chiusdano, where another vision told him to renounce material things.

Galgano objected that that would be as difficult as splitting a rock and to prove his point he struck one with his sword. The rock, it is said, yielded like butter.