The Prado presented the completed restoration of a work on Tuesday that it has concluded was a replica painted by one of Leonardo's apprentices alongside the Renaissance master in his workshop.

"This is the oldest and most important copy of the famous painting known to date," Gabriele Finaldi, the Prado's director of research and conservation, told reporters.

For years the copy, which reveals a younger and brighter version of the face that has captivated crowds for centuries, was believed to be one of dozens of replicas made long after Leonardo's death.

The painting has belonged to the Prado ever since the museum was founded in 1819 with the Spanish royalty's art collection, but with a background covered in black and a frame believed to be of oak, frequent in the work of northern European artists.

Experts began a technical study of the copy when the Louvre requested it two years ago for an exhibition opening next month. They found it had been painted on walnut, just like the Mona Lisa, and infrared images showed traces of background mountains just like in the original painting.

When conservators proceeded to remove a coat of varnish from the figure in the painting -- who is believed to be Lisa Gherardini, the wife of Florentine merchant Francesco Del Giocondo -- they uncovered important nuances of her 16th century dress and appearance.

They then stripped off the black paint, revealing a lush landscape offering important clues into Leonardo's original, which is covered in layers of dirt and cracked varnish.

While the origins of the Prado work are unknown, it may have arrived in Spain at the hands of Italian sculptor Pompeo Leoni during one of his frequent travels between Milan and Madrid in the 16th century, or by someone in the Spanish nobility. No records have been found to support either theory.

The painting will be on display at the Prado until March 13, when it will travel to the Louvre in Paris to sit next to the original for a temporary exhibition.

The Prado hopes the Louvre exhibition, which will show many works from Leonardo's studio, will offer more clues into which of the master's students is responsible for the painting.

"In Paris we'll be able to explore more on who the painting's supposed author was," Miguel Falomir, curator of Italian renaissance painting at the Prado said.

(Editing by Fiona Ortiz and Paul Casciato)

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