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Prior to sale the work had been declared an object of ‘great cultural interest’, and made subject to export restrictions.

The 22 x 17in (57 x 44cm) painting Retrato de niña o Joven Inmaculada (Portrait of a Girl or Young Immaculate) was recently discovered in an old Madrid family collection by consultant Richard de Willermin, a specialist in 17th and 18th century Italian, Flemish and Spanish art. He was convinced it was by Velázquez.

The auction house had been cautious not to release an estimate before the sale but bidding opened at €8m (£6.81m) with no higher bidding. It is understood to have sold to a Spanish collector.

Although little is known of Velázquez’s early life, it is thought the subject could be his sister with this picture painted around 1616-17 when the artist was still a teenager and living in Seville.

The pose of a young girl with hands held together in prayer is similar to that adopted in Velázquez's Immaculate Conception 1618-19 in National Gallery, London.

X-ray analysis undertaken by the auctioneers suggests that beneath underpaint the girl shares a crown of stars similar to that in the Immaculate Conception.

Specialists at the Prado Museum have looked at the painting, but the museum had not commented publicly on its findings in advance of the sale.

More on this story will appear in next week’s print edition.