Estimate, the 2013 Ascot Gold Cup winner belonging to Queen Elizabeth II, has tested positive for the banned substance morphine, Buckingham Palace announced July 22.

The development follows the news late last week that traces of morphine had been found in samples provided by five horses, Estimate among them. According to the Racing Post, it is thought the positive drug tests might have been a consequence of eating contaminated feed.

Estimate, a 5-year-old daughter of Monsun trained by Sir Michael Stoute, was presented with the Cartier Stayer Award in November.

In a statement reported by the Racing Post, 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