LONDON (REUTERS).- A horse painting by George Stubbs fetched 22.4 million pounds ($35.9 million) at Christie's in London on Tuesday, the third most valuable old master painting to be sold at auction, the company said.

"Gimcrack on Newmarket Heath, with a Trainer, a Stable-Lad, and a Jockey" had been expected to fetch 20-30 million pounds excluding fees. The sale price included buyer's premium.

The work was painted by Stubbs, renowned for his anatomically precise portrayals of horses, in 1765, and was 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.

Christie's said the result placed Stubbs third in the old master auction ranking behind "The Massacre of the Innocents" by Peter Paul Rubens, which fetched 49.5 million pounds at Sotheby's in 2002, and "Modern Rome - Campo Vaccino" by J.M.W. Turner which raised 29.7 million pounds at Sotheby's in 2010.

Also on Tuesday, Christie's set an auction record for Thomas Gainsborough, whose "Portrait of Miss Read, Later Mrs. William Villebois" sold for 6.5 million pounds ($10.5 million), nearly doubling the previous high.

However, another Gainsborough painting on offer, "Portrait of Colonel John Bullock," failed to find a buyer, according to Christie's website. It had been expected to fetch between 3.5 and 5.0 million pounds.

Overall, the old master and British paintings evening auction raised 49.9 million pounds including fees, versus pre-sale expectations of 49.5-74.2 million pounds excluding fees.

(Reporting by Mike Collett-White; editing by Patricia Reaney)