The Prado said it 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.

The painting is thought to have been created by one of Leonardo's students alongside the 16th century original.

There are dozens of surviving Mona Lisa replicas from the 16th and 17th centuries - when, as a new US exhibition illustrates, copying famous artworks was a thriving business.

The Art Newspaper, which reported the discovery, said the "sensational find will transform our understanding of the world's most famous picture".

The original painting, which currently hangs at the Louvre in Paris, is obscured by several layers of old, cracked varnish.

However, cleaning and restoration is thought to be too risky because the painting is fragile.

The Art Newspaper 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.

Martin Bailey, who reported on the discovery for the paper, told the BBC: "You see Lisa's eyes, which are quite enticing, and her enigmatic smile. It actually makes her look much younger."

In fact, the new painting has led experts to speculate that the woman who sat for the Renaissance Masterpiece was in her early 20s - much younger than the Louvre's original appears to show.

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.

The museum presented its findings at a conference on Leonardo da Vinci at London's National Gallery.

There is still some restoration to complete on the painting but, once it is finished, it will be exhibited at the Louvre in March, allowing visitors to compare the two works.

bbc.com