The Queen’s racehorse Estimate, which tested positive for morphine after finishing second in the Gold Cup at Ascot last month, may have got the drug into its system by eating poppy seeds.

A source told Daily Express writer David Pilditch, “There is a huge poppy field close to the [feed milling] plant. It could provide an explanation as to why the feed was contaminated.”

The horse could now be disqualified and the £80,625 prize money for coming second in the Gold Cup could be recalled.

The humiliating incident is likely to have provoked a furious reaction from the Queen.

The five-year-old mare is one of five horses from different stables who failed drugs tests in recent weeks.

Feed company Dodson & Horrell has faced suggetsions that one of its products, a contaminated batch of Alfalfa Oil Plus could be the culprit.

However, the Racing Post reports that trainer Eve Johnson Houghton - who admitted on Wednesday that her horse Charlie Wells had also tested positive - said although she uses Dodson & Horrell products, she has never fed her horses Alfalfa Oil Plus.

Given says: "I suspect it has been an issue with contaminated feed. Poppies are grown for a pharmacological purpose, so it could have been spread through problems with transportation, or if harvesting and storage equipment hasn't been cleaned properly.

"I suspect this isn't a common occurrence as I know feed companies operate stringent policies with their suppliers and insist that the equipment they use is properly treated."

Dodson & Horrell, horse and dog food manufacturer to the Queen since 2006, confirmed to the Express that contamination could have come from poppies, but added, “No positive test results have been recorded in any other Dodson & Horrell product currently in the market.”

Estimate made sporting history for the Queen, 88, by winning last year’s Gold Cup at Royal Ascot – the first monarch to have a winner in the event’s 207-year-history.

Footage of the Queen laughing, grinning and slapping her knees as she urged her horse on to victory were a high spot of the festival.