Horse Racing: Epsom Derby

Queen Elizabeth presents the Coronation Cup to Joseph O'Brien jockey of the winning horse Camelot of the Investec Derby at Epson Downs. (Leo Mason-USA TODAY Sports / June 2, 2012)

LONDON -- Queen Elizabeth's racehorse Estimate, the 2013 Ascot Gold Cup winner, has tested positive for the banned substance morphine, Buckingham Palace said on Tuesday.

Estimate was one of five horses under various trainers who were found by the British Horseracing Authority to be affected.

"On Thursday 17th July the British Horseracing Authority (BHA) announced that a number of post-race samples, obtained from recent race meetings, had been found to indicate the presence of morphine, which is a prohibited substance on race days," John Warren, the Queen's bloodstock and racing advisor, said in a statement.

"Initial indications are that the positive test resulted from the consumption of a contaminated feed product."

Warren added: "(Trainer) Sir Michael (Stoute) is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.

"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full co-operation."