A French art expert believes a charcoal drawing kept in a collection for more than 150 years may be a preparatory sketch by Leonardo da Vinci for the "Mona Lisa."

The black-and-white drawing of a woman nude from the waist up, known as the "Monna Vanna," was previously attributed to Leonardo's studio, suggesting it was done in his style by a pupil or follower, not by the master himself.

But after preliminary tests at the Louvre Museum, experts believe the sketch may well have been drawn by Leonardo.