LONDON, UK—JULY 23, 2014—Buckingham Palace confirmed Tuesday a championship-winning racehorse belonging to Britain's Queen Elizabeth has tested positive for morphine. The British Horseracing Authority (BHA) said that five horses tested positive for the banned substance, and the Queen's Gold Cup winner Estimate (TB) was one of them.

"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.

Morphine is classed as a "naturally occurring prohibited substance" and its use is banned on race days but allowed during training in the UK. Warren said initial indications suggested the positive test resulted from "the consumption of a contaminated feed product".

Renowned thoroughbred horse trainer Michael Stoute trains Estimate (TB).

"Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables," Warren added.

The Queen has been informed of the news.

Estimate (TB) won the Gold Cup at last year's Royal Ascot - one of the most respected racing meets in the world. The 5-year-old daughter of Monsun (TB) finished second in the 2014 Gold Cup.

While the mare's 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.

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.