Equine portrait set to become one of the most expensive British paintings ever sold

In British master George Stubbs's 1765 painting of the legendary English thoroughbred Gimcrack, the artist shows the horse several lengths ahead of its competitors. Now the work is expected to gallop into the record books to become one of the most expensive British paintings ever sold.

"He became something of a favourite, winning 28 out of 36 races which he was entered for, which was a remarkable feat."

The sale will make the work one of the most expensive Old Masters ever sold at auction. The current record for an Old Master is held by Flemish artist Sir Peter Paul Rubens, whose The Massacre of the Innocents sold for £50m in July 2002. If the Stubbs raises more than £20m it will make it the sixth most expensive Old Master ever sold, above Rembrandt's Portrait of a Lady Aged 62, which sold at Christie's for £20m in 2000.

"It is of great importance on many levels," said Mr Stainton. "The use of space is daring and unprecedented in British art at the time. It was a wonderfully wide panorama of the scene, and was unusual in that the central focus is shifted to the extreme left of the composition. In both pictorial and compositional senses it is groundbreaking."

Stubbs spent his early career as a portrait painter in his native Liverpool and York, before finding fame as a painter of horses. In 1754 the artist spent 18 months in a Lincolnshire farmhouse, where he dissected and drew horses. In 1766, he published a scientific study The Anatomy of a Horse, which added to Stubbs's reputation as the greatest artist-scientist since Leonardo da Vinci.