The Queen's horse Estimate has tested positive for morphine, Buckingham Palace have confirmed.
Morphine is a permitted medicine but is prohibited on race days. Palace officials believe the 2013 Gold Cup winner at Ascot was a victim of contaminated horse feed product.
The news comes a week after the British Horse Racing Authority announced that five unnamed horses, from varying stables, had tested positive for the substance.
The Queen and the Duke of Edinburgh will on Wednesday officially open the 20th Commonwealth Games in Glasgow at Celtic Park.