Estimate was one of five horses under various trainers who were found by the British Horseracing Authority to be affected.
The runner-up in this year’s Ascot Gold Cup in June, a year after winning the race, tested positive to the banned substance morphine.
The mare, who was beaten a neck by Leading Light in a gripping finish to this year’s renewal, will almost certainly be disqualified from second place.
As a result, the Queen will have to forfeit the £80,625 prize money Estimate won at Royal Ascot in June.
"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."
The British Horseracing Authority runs a near-to zero tolerance approach to the drug with this a very high-profile case.
The drug is allowed in training as either a pain killer or sedative but, at the levels found in these five horses, is not considered performance enhancing.
- Sports & Recreation
- British Horseracing Authority