A racehorse owned by the Queen has tested positive for morphine, a prohibited substance, Buckingham Palace announced.
Estimate, which won the Gold Cup at Royal Ascot in 2013 and came second in the same race this year, is a five-year-old filly trained by Sir Michael Stoute.
Initial indications are that the positive test resulted from the consumption of a contaminated feed product, the Queen's racing advisor said. The Queen, who cheered the horse on to success at Royal Ascot last year, has been informed of the development.
Although morphine is not prohibited during training, it is banned on race days.
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 last year when she became the first reigning monarch to win Royal Ascot's Gold Cup.
With a clap of her hands and a beaming smile, she celebrated the moment the horse finished first, as her family jumped for joy behind her in the royal box.
Princesses Beatrice and Eugenie had joined their grandmother for a day at the races and could not contain their excitement during the dramatic finish.
The Queen's smile lasted long after her horse crossed the finish line and she was still beaming when she joined jockey Ryan Moore and trainer Sir Michael Stoute in the winners' enclosure for the trophy presentation.
At the time Sir Michael said the Queen greatly appreciated the victory: ''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.''
The Queen was scheduled to hand out the winning trophies for the Gold Cup but the Duke of York stepped in to do the duties.
Prime Minister David Cameron also praised the Queen's win in a tweet: ''Huge congratulations to Her Majesty for Estimate's victory at Royal Ascot - first time ever a reigning monarch's horse has won the Gold Cup.''
The Queen won £155,960 in prize money as the owner.