Art experts in the United States have revealed they have found a hidden painting beneath one of Pablo Picasso's early masterpieces.

Using infra-red technology, they discovered a portrait of a bearded man wearing a jacket and bow tie and resting his hand on his cheek underneath Picasso's famous work The Blue Room.

Experts say it is likely the painter could not afford to buy a new canvas when he painted The Blue Room in 1901.

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 said 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.

It appears the hidden portrait - revealed in full only now - was painted just before The Blue Room was completed, with Picasso reportedly often painting over works in progress as his ideas changed.

It is not clear who the man in the hidden portrait is.