London (CNN) -- A previously unknown portrait by famed Spanish artist Diego Velázquez has been unveiled in London after it was spotted in a consignment of works by a largely forgotten British painter.
"Portrait of a Gentleman" came to light when a descendant of 19th century painter Matthew Shepperson put a series of paintings by the artist up for sale in August 2010.
As a work by Shepperson, the item was expected to sell for between $320 and $480.
But the auction was halted when keen-eyed experts at Bonhams' Oxford saleroom spotted similarities between it and other works by Velázquez -- one of the best-known of the "Old Masters."
Following more than a year of tests, x-rays and research, art historians have confirmed that the painting is in fact by the 17th century Spanish portraitist.
The change of attribution means it is likely to sell for between $3.2m and $4.8m -- 10,000 times its original estimate.
Auction house staff say the painting's owner was stunned at the news.
Velázquez experts Dr Peter Cherry, Professor of Art History at the University of Dublin, and Carmen Garrido, Head of Technical Services at the Prado Museum in Madrid were among those to study the picture.
Writing in the Spanish arts magazine ARS, Cherry said: "the particularized likeness and recognisably lifelike texture, weight and colours of the fleshy face speak of the actual encounter between subject and painter; while the style and technical brilliance of the representation itself betrays its author."
The identity of the painting's subject is not known, but it is thought the balding, bearded man in a stiff, starched collar may be Juan Mateos, King Philip IV of Spain's master of the hunt.
The Velázquez portrait will now go on display to the public at Bonhams London saleroom on Sunday December 4, ahead of the company's Old Master Paintings auction on Wednesday December 7, where it is expected to be the star lot.
"Velázquez is one of the greatest geniuses in the entire history of Western art," said Bonhams' Old Masters expert Andrew Mckenzie.
"The discovery of this lost treasure is a once-in-a-lifetime experience -- it is tremendously exciting to be able to bring it to the world's attention.
"We expect there to be great interest from around the globe as works by this master so rarely come to auction."