A horse owned by Britain's Queen Elizabeth II which won one of England's most prestigious races has failed a drugs test, Buckingham Palace announced.
Estimate, which lifted the Gold Cup at Royal Ascot in 2013 and came second in this year's edition, has tested positive for morphine, a banned substance.
A statement by the Queen's racing advisor said initial indications were the positive test had resulted from the "consumption of a contaminated feed product".
The Queen, renowned for her love and knowledge of horse racing, cheered on Estimate -- whose trainer Michael Stoute has been a leading figure on the English turf scene for more than 30 years -- when the now five-year-old filly won at Ascot last year.
A statement by John Warren, the Queen's bloodstock and racing advisor, said: "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.
"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 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 in 2013 when she became the first reigning monarch to own a Gold Cup-winning horse.
An often reserved figure in public, the 88-year-old monarch was seen smiling as Estimate crossed the winning line in first place as her family, including granddaughters Princesses Beatrice and Eugenie, were seen jumping for joy behind her in the royal box.
Immediately after Estimate's victory, Stoute said: "It's a special thrill to win this race for the Queen and it will have given her enormous pleasure -- she really loves this game and it's a great recreation for her.
"She told me it was a very big thrill and thanked everybody involved."