The Queen’s Group One-winning racehorse Estimate has tested positive for morphine.

Buckingham Palace confirmed the five-year-old mare tested positive for the banned substance in a statement last night (Tuesday 22 July).

Estimate finished second in last month’s Gold Cup at Royal Ascot — a race she won last year for trainer Sir Michael Stoute.

The horse faces disqualification from this year’s race and will forfeit the £80,625 prize money.

The Palace said that initial indications are that the positive result has come from the consumption of contaminated feed. If this is the case Sir Micheal is unlikely to face any punishment.

Last week the British Horseracing Authority (BHA) revealed that traces of morphine had found in post-race samples from five racehorses.

Morphine is listed by the BHA as a “naturally occurring substance”. It is permitted out of competition but banned on racedays.

Trainer Charlie Hills confirmed he also trains one of the five positive horses.

John Warren, the Queen’s bloodstock and racing adviser, said in a statement: “On Thursday, 17 July, 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, which is a prohibited substance on race days. Five horses, under the care of various trainers, were affected.

“I can confirm that one of those horses was Estimate, the five year-old filly trained by Sir Michael Stoute and owned by the Queen. 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 cooperation. There will be no further comment until the BHA announces its considered findings. Her Majesty has been informed of the situation.”

In 2002 the Willie Mullins-trained Be My Royal was disqualified after the Hennessy Cognac Gold Cup for testing positive for morphine. Following that case a code of practice was drawn up by feed manufacturers to limit the chances of contamination.