Summers Place Auctions said the female Diplodocus longus skeleton, nicknamed "Misty," measures 55 feet (17 meters) long and 19 feet (6 meters) tall. It said it was found mostly intact near a quarry in Wyoming by accident, and the skeleton was treated at a fossil laboratory in Holland before being assembled in Britain.
Author Errol Fuller, who curates the auction, said there are about six such skeletons in museums around the world, including those in Pittsburg and Washington.
The auction house, in southern England's West Sussex, said Wednesday the skeleton would go under the hammer on Nov. 27.
The sellers say it is likely to sell for more than 400,000 pounds ($640,000).