It had long been suspected that another painting lay behind The Blue Room, one of the Spanish artist's key pieces from his "blue" period in the first few years of the 20th Century.
Reports say the idea was first raised in the 1950s, but it was not until 40 years later that X-ray technology revealed a fuzzy image beneath the famous portrait, which depicts a woman bathing.
Using infra-red technology, experts in the United States discovered a portrait of a bearded man wearing a jacket and bow tie and resting his hand on his cheek underneath the famous work.
Experts say it is likely the painter could not afford to buy a new canvas when he painted The Blue Room in 1901.