A championship-winning racehorse belonging to Britain's Queen Elizabeth has tested positive for morphine, Buckingham Palace confirmed on Tuesday.
The British Horseracing Authority (BHA) revealed that five horses tested positive for the banned substance, and Estimate – which won the prestigious Gold Cup at Royal Ascot last year - was one.
"The British Horseracing Authority announced that a number of post-race samples, obtained from recent race meetings, had been found to indicate the presence of morphine… I can confirm that one of those horses was Estimate," John Warren, the Queen's bloodstock and racing advisor, said in a statement.
Read More'World Cup' of horseracing gets 5,000 bets a minute
Morphine is classed as a "naturally occurring prohibited substance" and its use is banned on race days but allowed during training. Warren said initial indications suggested the positive test resulted from the "the consumption of a contaminated feed product".
Estimate is trained by renowned thoroughbred horse trainer Michael Stoute, and Warren added: "Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables."
The Queen has been informed of the news.
Read MorePrince Charles spent $433K on Mandela funeral trip
Estimate, a five-year old mare, won the Gold Cup at last year's Royal Ascot - one of the most respected racing meets in the world. This year, the Queen's horse came second.
While its Gold Cup victory is not under threat, the £80,625 ($137,458) runner-up prize money from this year's event may have to be forfeited as a result of the positive test.
The horse's trainer Stoute is unlikely to face any penalty if the investigation finds that the morphine was ingested inadvertently.
Morphine is a naturally occurring substance found in flowers like poppies. It can contaminate growing crops, and as such, end up in animal feed.