(CNN) -- The earliest known copy of Leonardo da Vinci's "Mona Lisa" -- thought to have been painted at the same time as the original masterpiece -- has been discovered at the Prado Museum in Madrid, Spain.
The work offers art-lovers a tantalizing hint of what the model for the world's most famous painting really looked like.
Conservators found the portrait hidden beneath layers of black overpainting during restoration work on a picture initially thought to have been a later replica of the "Mona Lisa."
The restored version shows the same woman that Leonardo depicted, against a landscape similar to that shown in the background of the original, which now hangs in the Louvre in Paris.
And while the features of Leonardo's subject have been dulled by centuries of dirt and layers of cracked varnish -- which are unlikely ever to be removed -- in the recently-rediscovered copy, she appears fresher faced and younger than her better-known "twin."
"This sensational find will transform our understanding of the world's most famous picture," the Art Newspaper reported, adding that the underdrawing found on the Madrid version "suggests that the original and the copy were begun at the same time and painted next to each other, as the work evolved."
Miguel Falomir, curator of Italian painting at the Prado, told a press conference at the museum expert analysis suggested a strong link between Leonardo and the artist who painted the copy.
"The painting was done in the painter's own workshop," he was quoted by AFP as saying.
"It is absolutely consistent with Leonardo's work," he said, but he added: "It is a work in which Leonardo himself did not intervene."
The painting is expected to be unveiled to the public at the Prado Museum later this month.
It will then go on display at the Louvre in March, as part of the "Leonardo's Final Masterpiece" exhibition, which focuses on his work, "The Virgin and Child with St. Anne."