A painting sold at auction for £75,000 three years ago could be worth millions after experts authenticated it as a work by Italian master Caravaggio.
Art dealer Clovis Whitfield said he made the discovery when he examined Apollo the Luteplayer for a client.
It had been sold at Sotheby's in New York in 2001, where the catalogue listed it as the possible work of 17th Century artist Carlo Magnone.
But Sotheby's remains "adamant" that the painting is not by Caravaggio.
A spokesperson for the auction house said they had consulted a number of eminent art experts prior to the sale in January 2001 - where it fetched $110,000 (then £75,000) - and that their position had not changed.
Mr Whitfield, an expert in Italian 17th century art, told BBC News Online his interest in the painting was sparked when he saw it in the auction catalogue.
It was thought to be a copy of a Caravaggio painting that hangs in the Metropolitan Museum of Art in New York, and a similar version of the same painting in Russia's Hermitage Museum.
But Mr Whitfield said a 17th Century description of a Caravaggio painting by biographer Giovanni Baglione corresponded with Apollo the Luteplayer, making it worthy of investigation.
It was not until he managed to examine the painting that the possibility of it being a work by Caravaggio came to light.
"It seemed to be the real McCoy but it was only after we got it home, cleaned it and took x-rays that we discovered there were changes that suggested it was an original rather than a copy," said Mr Whitfield.
"Various adjustments, such as to the profile of the boy's hands, had been made and there were incisions that are characteristic of Caravaggio.
"That would not have been done if it was a copy."
Mr Whitfield went on to say he believed that the painting was the original, with the Hermitage piece a second original.
His findings have been backed by eminent art historians, including Sir Denis Mahon who is considered to be the leading expert on 16th and 17th century Italian art.
Sir Denis originally said the painting was not by Caravaggio, but changed his mind after it was cleaned.
But the spokeswoman for Sotheby's said a number of experts had attributed the work to Magnone.
She added: "We have not learned anything since the sale that would change our original cataloguing and we remain completely confident with our cataloguing and with our research.
"Were we to take the painting in for sale today, we would definitely not catalogue it as by Caravaggio."