Conservators at Madrid's Prado museum have identified what they believe is the earliest copy of Leonardo da Vinci's Mona Lisa.
Brighter faced and younger than the original which hangs in the Louvre in Paris, the lady in the portrait has long been standing against a black background.
Art historians thought it was just one of dozens of replicas produced in the centuries after Leonardo's death.
But as paint layers were stripped away during recent restoration work, a landscape much similar to the original backdrop in Da Vinci's masterpiece, emerged.
NEWS: Nude, Mona Lisa-Like Painting Surfaces
Intrigued, the curators turned to infrared reflectography, a technology which enables to see beneath the painted surface. They compared images obtained in 2004 from the original Mona Lisa with the Madrid copy.
"In the under-drawing you can see changes which are only apparent underneath the surface of the Louvre painting," said Gabriele Finaldi, deputy director of conservation at the Prado Museum.
The discovery suggests the picture was being produced at the same time as Leonardo was painting his masterpiece.
NEWS: Was Mona Lisa a Dude?
"The artist of this picture was making the same changes Leonardo was introducing into the original," Finaldi said.
Listed in the 1666 inventory of Madrid's Alcazar Palace, the painting, which is close in size to the original Mona Lisa, enjoyed much attention in the past. Attributed to Leonardo himself, it was copied by various artists, such as Gaspare Sensi, known as Gaspar Sensi y Baldachi (1794-1880).
According to Alessandro Vezzosi, director of the Museo Ideale in Vinci, where Leonardo was born in 1452, the discovery is extremely important.
The curator of the exhibition Leonardo Da Vinci and His Idea of Beauty, which will open in March in Tokyo, Japan, with more than 20 Mona Lisa inspired works, Vezzosi believes that the author is a Spanish artist.
BLOG: The Story Behind the Mona Lisa Heist
Indeed, many Da Vinci inspired artworks spread in Spain at that time.
"Leonardo had several Spanish apprentices, and one of them was in Florence at the beginning of 1500. He was called Ferrando, and has been identified either as Fernando Yáñez de la Almedina or in Fernando Llanos," Vezzosi told Discovery News.
Mentioned in Leonardo's "Manuscript H," Ferrando is also recorded in a 1505 document which enlists the artists working with the master at the now lost fresco The Battle of Anghiari.
"It says that 'Ferrando the painter' was paid 5 fiorins, while Tommaso Masini, also known as Zoroastro da Peretola, was paid 1 fiorin to crush the colors," Vezzosi said.
Expected to be unveiled at the Prado museum later this month, Mona Lisa's earliest copy will then be lent to the Louvre as an addition to the exhibition Leonardo's Last Masterpiece: The Sainte Anne.
Image: Detail of Mona Lisa's earliest copy. Credit: Museo Nacional del Prado.