Originally catalogued as a 'Portrait of a Gentleman' and attributed to a minor 19th century British artist, the 18½ x 15½in (47 x 39cm) oil on canvas portrait, was brought to the attention of the auctioneers' Old Master department in London and will now be the highlight of their auction in New Bond Street on December 7 with a £2m-3m estimate.
It was part of a consignment of pictures from a direct descendant of the artist Matthew Shepperson (1785-1874). He was best known as a copyist - his version of John Hoppner's portrait of Nelson, for instance, is in the National Maritime Museum in Greenwich.
The other works consigned to Bonhams included pictures by Shepperson which each made under £500 at the Oxford sale in August 2010, but the saleroom's specialists soon realised this portrait was in fact a 17th century picture of outstanding quality.
The auctioneers' research over the next 14 months has now led to a confident attribution to Diego Rodriguez de Silva y Velázquez (1599-1660), which has been confirmed by Dr Peter Cherry, one of the authorities on the Spanish master and his school, and by Carmen Garrido, head of technical services at the Prado Museum in Madrid.
Bonhams' director of Old Master Paintings Andrew McKenzie said: "It is wholly in the artists' hand and is in as good condition as you could ask for."
