Mona Lisa's 'twin sister' found 500 years later

London -  Spanish curators claim the 'Mona Lisa' at the Prado in Madrid was executed by an artist in Leonardo da Vinci's workshop at the same time as the original.

It is the first known copy of the most famous painting in history, and a discovery that curators at Spain's national art museum believe sheds new light on the creation of the masterpiece, The  Independent reported Thursday.

"It's as if we were standing in the workshop itself, and at the next easel you can see that the artist was working step by step with Leonardo. When Leonardo made a change, he made a  change," the newspaper quoted deputy conservator Gabriele Finaldi as saying.

The copy sits in a dimly-lit room awaiting the finishing touches of a two-year restoration, during which its true origin was revealed.

Curators decided it needed a face-lift because it was going on loan to the Louvre in March. Following X-ray and infrared studies, they were surprised to find a landscape hidden beneath the  dark paint behind the subject, according to the newspaper.

Conservators believe the artist could be Francesco Melzi, one of Leonardo's favourite pupils. "When you look at the copy, you can imagine that this is what the 'Mona Lisa' looked like in the  16th century," Finaldi said.

The 'Mona Lisa' is widely believed to be a portrait of Lisa Gherardini, a Florentine merchant's wife, and the copy makes her look younger and more seductive.

Miguel Falomir, chief conservator for renaissance painting, said: "When the X-ray revealed the landscape, we saw it was in absolutely extraordinary condition."

"It was the most surprising thing to emerge in the conservation workshop in the 14 years I've been at the Prado."

The copy has belonged to the museum at least since the 1666, first as part of the royal collection and then as a state treasure. It was first thought to have been produced by a Flemish hand  after da Vinci's death. Then it was believed to be a later Italian copy.

The Art Newspaper, which first published the remarkable discovery, said: "This sensational picture will transform our understanding of the world's most famous picture."