A five-year-old filly named "Estimate" owned by Britain's Queen Elizabeth II that won the Gold Cup at Royal Ascot last year has tested positive for morphine.

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The racehorse Estimate showed presence of a banned painkiller known as morphine in its 'A' sample sent to the British Horseracing Authority (BHA) last week announced by the monarch's bloodstock and racing adviser, John Warren Tuesday.

In the tests conducted by the BHA, five racehorses from various trainers were confirmed of having the banned painkiller in their bloodstreams and one of them is the Queen's racehorse.

Buckingham Palace said that early indication suggests that the banned substance may have been mixed in the feed consumed by Estimate.

Warren released a statement saying that Estimate's trainer Sir Michael Stoute and the feed company involved are already conducting an investigation on how the illegal substance may have been contaminated in their products prior to its delivery in the stables.

"Her Majesty has been informed of the situation," said Warren.

Estimate, a female bay filly and ridden by jockey Ryan Moore, has won the Queen's Vase at Royal Ascot as a three-year-old. She also won the Sagaro Stakes as a four-year-old last year before winning again in the Gold Cup.

It was customary for the Queen to present the trophy to the winner, but since the Queen herself was the owner of the winning horse, her son Prince Andrew took place in presenting the winner.

This was the first time in the more than 200 years of the Gold Cup, that the reigning monarch has won the prestigious cup.

In June, Estimate competed again in the Gold Cup but finished second to "Leading Light", a bay horse trained by Aiden O'Brien, in a closely contested finish where Leading Light won by a neck.

The tests conducted last week was part of the post race drug tests.

TagsEstimate, Royal Ascot, horseracing, Ryan Moore, Leading Light, the Gold Cup, morphine