The Mona Lisa at the Prado in Madrid was thought to be just another fine copy, with added eyebrows and an odd black background.
But curators at Spain's national art museum yesterday announced a startling discovery: the painting was actually 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 perhaps the most famous painting in history and a discovery that curators believe sheds new light on the creation of the enigmatic masterpiece.
Deputy conservator Gabriele Finaldi said: "It's as if we were standing in the workshop itself and at the next easel," before adding: "You can see that the artist was working step by step with Leonardo. When Leonardo made a change, he made a change."
The simultaneous copy sits in a dimly-lit storage room awaiting the finishing touches of a two-year restoration, during which its true origin was revealed. Curators decided it needed a facelift because it was going on loan to the Louvre for an upcoming exhibit in March. Following X-ray and infrared studies, they were surprised to find a landscape hidden beneath the dark paint behind the subject.
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," Mr Finaldi continued. "It's not just the details and the colour use. It has also been protected from light and dirt for centuries. So what you see if a very reliable appearance."
Miguel Falomir, chief conservator for renaissance painting, said: "When the X-ray revealed the landscape, we saw it was in absolutely extraordinary condition." He continued: "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 Prado Museum at least since 1666.