Staff at an auction house near Kidlington were told a painting they were going to sell by a little-known 19th century artist was worth no more than £300.

But after art expert Lizzie Hill thought there was something amiss, it was discovered it was an Old Master painting by Diego Velazquez that is expected to sell for up to £3m.

Mrs Hill, department director of pictures at Bonhams in Shipton-on-Cherwell, near Kidlington, had been told the portrait was by the little-known Matthew Shepperson. But she thought she could see something different about the way the portrait had been painted, compared to 11 others by Shepperson.

As a result, she referred the portrait for a second opinion to the Old Master department at Bonhams in London. And her suspicions proved correct when the painting was confirmed as the work of Spanish painter Velazquez, who died in 1660.

The painting, thought to be a portrait of Juan Mateos, Philip IV’s Master of the Hunt, is now being auctioned at Bonhams in London on December 7 and the bidding is likely to exceed the estimate of £2m to £3m.

Mrs Hill who made her once-in-a-lifetime discovery in August last year, said: “This is very exciting and not something that happens every day in the art world.

“I thought there was a slim chance that the painting might be by a significant artist but I can’t say I thought it was a Velazquez.”

Mrs Hill, who lives near Banbury, studied history of art at London’s University College before completing an MA at the Courtauld Institute of Art in London.

Velazquez (1599-1660) was the leading artist in the court of King Philip IV of Spain.