Lisa Gherardini, the wife of a wealthy Florentine merchant, Francesco del Giocondo, has long been seen as the most likely model for the sixteenth-century painting.

But art historians have often wondered whether the smiling woman may actually have been da Vinci's lover, his mother or the artist himself.

Now experts at the Heidelberg University library say dated notes scribbled in the margins of a book by its owner in October 1503 confirm once and for all that Lisa del Giocondo was indeed the model for one of the most famous portraits in the world.

"All doubts about the identity of the Mona Lisa have been eliminated by a discovery by Dr. Armin Schlechter," a manuscript expert, the library said in a statement on Monday.

Until then, only "scant evidence" from sixteenth-century documents had been available. "This left lots of room for interpretation and there were many different identities put forward," the library said.

The notes were made by a Florentine city official Agostino Vespucci, an acquaintance of the artist, in a collection of letters by the Roman orator Cicero.

The comments compare Leonardo to the ancient Greek artist Apelles and say he was working on three paintings at the time, one of them a portrait of Lisa del Giocondo.

Art experts, who have already dated the painting to this time, say the Heidelberg discovery is a breakthrough and the earliest mention linking the merchant's wife to the portrait.

"There is no reason for any lingering doubts that this is another woman," Leipzig University art historian Frank Zoellner told German radio. "One could even say that books written about all this in the past few years were unnecessary, had we known."

The woman was first linked to the painting in around 1550 by Italian official Giorgio Vasari, the library said, but added there had been doubts about Vasari's reliability and had made the comments five decades after the portrait had been painted.

The Heidelberg notes were actually discovered over two years ago in the library by Schlechter, a spokeswoman said. Although the findings had been printed in the library's public catalogue they had not been widely publicised and had been received little attention until a German broadcaster decided to do some recording at the library, she said.

The painting, which hangs in the Louvre in Paris, is also known as "La Gioconda" meaning the happy or joyful woman in Italian, a title which also suggests the woman's married name.