Fresh proof has been found to support the theory that Leonardo da Vinci's Mona Lisa, seen here at the Louvre, was the wife of a Florence merchant, the head of the library of Germany's Heidelberg University said on Tuesday.

Germany's University of Heidelberg says a researcher has uncovered evidence that apparently confirms the identity of the woman behind the Mona Lisa's famous smile.

She is Lisa del Giocondo, wife of Florentine businessman Francesco del Giocondo, according to notes written in the margins of a 1503 book by a friend of Leonardo da Vinci as the artist worked on the masterpiece, the school said in a statement.

The discovery by a Heidelberg University library manuscript expert appears to confirm what has long been suspected.

Del Giocondo was first named as the likeness in the painting by Italian writer Giorgio Vasari in 1550, who also dated the work at between 1503 and 1506, the university said.

But because Vasari relied on anecdotal evidence, there was always doubts about the identification, and Leonardo is not known to have made any notes about the model's identity himself.

"One possibility discussed is the presentation of a fictitious likeness of a woman; Leonardo's female ideal," the school said.

But the finding by Heidelberg library expert Armin Schlechter settles the matter, according to the university.

In a copy of the works of Roman philosopher Cicero, Agostino Vespucci, a Florentine official and friend of Leonardo's, wrote in the margins that da Vinci was working on a portrait of Lisa del Giocondo. He dated his notes October 1503, also helping pin down the exact time Leonardo was working on the painting.

"All doubts as to the identity of the Mona Lisa are eliminated (by) one source," the university said.

The discovery was actually made in 2005, but was not widely known until a German radio station last week aired it in a report.