A horse owned by Britain's Queen Elizabeth II which won one of the world's most prestigious flat races has failed a drugs test, according to a royal spokesperson.

Estimate, which lifted the Gold Cup at Royal Ascot in 2013 and came second to Leading Light in this year's edition, 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, known 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 17 July the British Horseracing Authority 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."

Mr Stoute is believed to be working with the feed company in question to discover how the product may have become contaminated prior to delivery to his stables.

Mr Warren added that the queen has been informed of the situation and that no further comment will be made until the BHA announces its findings.