A racehorse owned by the Queen, which won the Gold Cup at Royal Ascot last year, has tested positive for a banned substance, Buckingham Palace announced on Tuesday.
Initial results indicate that Estimate tested positive for morphine after she consumed contaminated food, the palace said.
While morphine is a permitted medication, it is prohibited on race days.
The five-year-old filly trained by Sir Michael Stoute saw the Queen become the first reigning monarch to win Royal Ascot's Gold Cup. In this year’s race, Estimate came second to the winner Leading Light.
The Queen has been informed following an announcement by the British Horseracing Authority last Thursday, according to John Warren, the Queen's Bloodstock and Racing Advisor.
Mr Warren said in a statement that a number of post-race samples taken from recent race meetings "had been found to indicate the presence of morphine".
"Five horses, under the care of various trainers, were affected," he said.
He added that Sir Michael is “working closely” with the feed company involved to uncover how morphine may have contaminated the food prior before it was delivered to Estimate’s 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," he added.
At the time of Estimate’s win, 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," he added.
The Queen won £155,960 in prize money as the owner.