Nude 'Mona Lisa sketch' discovered in France

Experts say Leonardo da Vinci likely worked on the nude portrait

Leonardo da Vinci, who painted the Mona Lisa, died in 1519

For more than 150 years, the Conde Museum at the Palace of Chantilly in France has kept hold of an ambiguous charcoal sketch.

Featuring a nude woman looking over her left shoulder, the artwork — known as Monna Vanna — has only ever been attributed to Leonardo da Vinci's studio.

Following tests at the Louvre Museum in Paris, one French art expert has claimed the famous artist himself likely worked “at least in part” on the sketch.

They also say the subject of Monna Vanna may very well have been Lisa Gherardini, the same woman the Mona Lisa is believed to portray.

"The drawing has a quality in the way the face and hands are rendered that is truly remarkable,” curator Mathieu Deldicque told AFP.

"It is not a pale copy. We are looking at something which was worked on in parallel with the Mona Lisa at the end of Leonardo's life. It is almost certainly a preparatory work for an oil painting.”

Deldicque said the main clues linking the two paintings are the almost identical hands, the portraits being the same size, and small holes on the canvas suggesting it may have been traced onto another canvas.

Another art expert, Bruno Mottin — the Louvre’s conservation expert — confirmed the sketch dates from da Vinci’s lifespan and was of "very high quality”.

"We must remain prudent,” Deldicque added. "It is job that is going to take some time. It is a very difficult drawing to work on because it is particularly fragile."