MADRID.- Crowds are gathering at Madrid's Prado Museum to view a copy of the "Mona Lisa" for the first time since a restoration revealed it was almost certainly painted by one of Leonardo da Vinci's apprentices as he worked on the original.

The painting was put on display at the museum on Tuesday, where it will stay until it moves to Paris' Louvre museum next month to hang alongside the original as part of an exhibition on da Vinci's work.

Although the precise author of the copy has not been determined, both the Prado and Louvre believe it is probably the earliest known copy of "La Gioconda."

The copy has been part of Spain's art collection for hundreds of years. It had previously been on display in the Prado but no one paid much attention as it was considered a mediocre copy.

La Gioconda was painted by a pupil or follower of the artist at the same time as the original. The importance of this discovery, which was made during the study and restoration of the painting at the Prado for its inclusion in the exhibition at the Louvre on Leonardo that opens on 29 March, lies in the fact that as a contemporary and perfectly preserved copy, it contributes important information on both the landscape background and on numerous details of the mysterious sitter. The latter include the shape of the chair, the ornamentation of the cloth covering her breast and the semi-transparent veil around her shoulders.

Following its rediscovery, this copy of La Gioconda in the Museo del Prado, which has now been confirmed as a work of one of Leonardo’s pupils or followers working in his studio while the original was being painted, has not only been confirmed as the oldest known copy of this enigmatic image but also acquires considerable importance for its potential to cast more light on the Louvre’s painting.

Having previously been in the Spanish royal collections, the present copy entered the Prado when the museum was founded in 1819. Although the exact date and manner in which it entered the royal collections is unknown, it is probably the work that is referred to in 1666 in the inventory of the Alcázar as a female portrait associated with Leonardo.

This painting will be exhibited until the 13th March.