The Queen's Group One winner Estimate has been named as one of five horses in Britain to test positive for morphine.
The winner of the Gold Cup at Royal Ascot in 2013 and second in the same race this year, Estimate is trained by Sir Michael Stoute.
The British Horseracing Authority announced last week that five horses under the care of various trainers had shown the presence of morphine in their 'A' samples.
Initial indications are that the positive test taken from Estimate resulted from the consumption of a contaminated feed product, the Palace said.
Although morphine is not prohibited during training, it is banned on race days.
"On Thursday July 17 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. Five horses, under the care of various trainers, were affected," John Warren, the Queen's bloodstock and racing advisor, said.
"I can confirm that one of those horses was Estimate.
"Initial indications are that the positive test resulted from the consumption of a contaminated feed product.
"Sir Michael 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. There will be no further comment until the BHA announces its considered findings. Her Majesty has been informed of the situation."
Estimate made sporting history for the Queen last year when she became the first reigning monarch to win Royal Ascot's Gold Cup.
Prominent trainer Charlie Hills has revealed one of the five horses to test positive is in his yard.
One of Britain's major feed companies issued a statement last week that one of its suppliers had informed them of a possible component contamination.