LONDON (REUTERS).- Christie's will offer a George Stubbs horse painting for sale in London on July 5 and expects it to make more than 20 million pounds ($33 million), potentially putting it in the Old Masters auction elite.

"Gimcrack on Newmarket Heath, with a Trainer, a Stable-Lad, and a Jockey" was painted by Stubbs, renowned for his anatomically precise portrayals of horses, in 1765, and is described by the auctioneer as "a true masterpiece."

It depicts Gimcrack, one of the most admired 18th century racehorses which won 28 of his 36 races and finished unplaced only once.

The painting shows Gimcrack twice -- in the background winning a "trial" and in the foreground with his trainer and jockey, a stable lad rubbing him down.

Should the painting exceed pre-sale expectations, it would place Stubbs among the most valuable Old Master painters in auction history, according to Christie's.

Although exchange rates fluctuate, the most expensive Old Master painting at auction remains "The Massacre of the Innocents" by Peter Paul Rubens which fetched 49.5 million pounds at Sotheby's in 2002.

That is followed by two works by J.M.W. Turner, a painting by Pontormo and, in fifth place, Rembrandt's "Portrait of a man with arms akimbo" which sold for 20.2 million pounds at Christie's in 2009. The list does not include drawings.

"Painted the year before James Christie oversaw our company's first auction, the sale of this masterpiece will be a cornerstone moment in the history of Christie's," said Richard Knight, international co-head of Old Master and 19th century art at Christie's.

"It is telling that as a result of the global nature of the 21st century art market, Stubbs, a very British artist, is set to join a small and select group who represent the most valuable old master artists ever sold, placing him alongside Raphael, Rubens, Rembrandt, and Turner."

(Reporting by Mike Collett-White, editing by Paul Casciato)

© Thomson Reuters 2011. All rights reserved.