An early replica of Leonardo Da Vinci's Mona Lisa has been discovered at the Prado Museum in Madrid.
The Prado said it did not realise its significance until a recent restoration revealed hidden layers.
The artwork features the same female figure, but was covered with black paint and varnish.
The painting is thought to have been created by one of Leonardo's students alongside the 16th century original.
The BBC reports there are dozens of Mona Lisa replicas from the 16th and 17th centuries.
The original painting, which currently hangs at the Louvre in Paris, is obscured by several layers of old, cracked varnish.
The discovery was reported by the Art Newspaper, which said the removal of the black paint on the replica had revealed ''the fine details of the delicate Tuscan landscape'', which mirrors the background of Leonardo's masterpiece.
As the replica remained hidden for so long under the overpaint, experts had believed it was painted long after Leonardo's death.
But after using x-rays to analyse the original drawings underneath, conservators have concluded the work was carried out at the same time as Leonardo's original by one of his pupils.
The museum presented its findings at a conference on Leonardo da Vinci at the National Gallery in London.