Researchers in Italy claim to have unearthed the portrait of a noblewoman by Leonardo da Vinci which has been lost for 500 years and features the same enigmatic smile as his Mona Lisa.

The portrait of Isabella d'Este, which carbon dating suggests was painted around the start of the 16th century, has been found in a vault in a private collection in Switzerland, and has been verified by a leading authority on the renaissance polymath.

"There are no doubts that the portrait is Leonardo's work," said Carlo Pedretti, an emeritus professor of art history at the University of California.

If acknowledged as genuine - and if experts concur that it was painted before the Mona Lisa - the portrait could shake up academic studies of one of the world's most famous paintings.