The earliest copy of Leonardo da Vinci's masterpiece, Mona Lisa, has been found in the vaults of a Spanish museum, looking younger and more ravishing than the original.
Art historians have hailed the discovery, made during conservation work at the Prado Museum, as one of the most remarkable in recent times.
Museum officials said it was almost certainly painted by one of Leonardo da Vinci's apprentices alongside the master himself as he did the original.
Replica: Painted alongside the original (left), historians say the copy (right) gives another insight into what the model for one of the world's most famous paintings actually looked like
Rare discovery: Employees of Madrid's Prado Museum stand next to the authenticated contemporary copy of Leonardo da Vinci's Mona Lisa
It is not the Mona Lisa, but you might think of her as Mona Lisa's sister, who - after more than five centuries - is finally having her debutante party.
Painted alongside the original, historians say it gives another insight into what the model for one of the world's most famous paintings actually looked like.
The copy has been part of the Prado collection for years but officials said they did not realise its significance until a recent restoration revealed hidden layers.
The artwork features the same female figure, but had been covered over with black paint and varnish.
Two years ago, to get the copy ready for a da Vinci exhibition to be held in Paris this year, tests were done and restorers discovered something hidden under the black coat.
When the black covering was removed, a Tuscan landscape very similar to the one in the original emerged.
Unveiled: The copy has been part of the Prado collection for years but officials said they did not realise its significance until a recent restoration revealed hidden layers
The Prado painting was long thought to be one of dozens surviving replicas of the masterpiece made after Leonardo's death but it is now believed to have been painted by one of his key pupils, Francesco Melzi, working alongside the master.
Prado's technical specialist, Ana González Mozo, said: 'It is quite possible that Leonardo's assistant met Lisa and may even have been present when she sat for the master.
'She may also have come to the studio when finishing touches were being applied to the face in the painting.'
Different perspective: The copy gives art lovers and experts a chance 'to admire the Mona Lisa with totally different eyes'
Ms Mozo said the underdrawing of the Madrid replica was similar to that of the original, which suggests both were begun at the same time and painted next to each other, as the work evolved.
The Louvre original, displayed behind glass, is obscured by cracked darkened varnish, making the woman appear middle aged. Because of its fragility, cleaning and restoration is thought to be too risky.
But art historians believe the Prado's Mona Lisa which is in the process of being painstakingly stripped of a dark over-paint reveals her as she would have looked at the time- as a radiant young woman in her early 20s.
Miguel Falomir, the Prado's director for Italian painting, said the copy gives art lovers and experts a chance 'to admire the Mona Lisa with totally different eyes.'
Besides the black background, one other difference from the original is the woman in the copy has eyebrows and the Mona Lisa in the real masterpiece does not.
There are dozens of the surviving replicas of the masterpiece from the 16th and 17th centuries.
The sitter is generally believed to represent Lisa Gherardini, the wife of the Florentine cloth merchant Francesco del Giocondo and is thought to have been painted between 1503 and 1506.
After five hundred years, the two versions will be reunited again later this year.
The Prado plans to put it on display later this month before it travels to the Louvre for the da Vinci show, giving specialists and visitors the first chance to compare the two works.