Eve Johnson Houghton is the latest trainer to come forward and reveal one of her horses has tested positive for morphine in a case which has involved The Queen.

The Oxfordshire trainer has confirmed three-year-old gelding Charlie Wells failed a drugs test after winning at Chepstow on June 23.

She said she was mystified as to where the contaminated food was contained and has had all of her string tested as a precaution.

"I had all my horses tested, but they haven't come up with the results yet," Johnson Houghton said.

"I don't know which bit of feed it's in, but hopefully we'll get to the bottom of it, and hopefully it's a one-off."

Johnson Houghton admitted to using some products provided by animal feed manufacturer, Dodson & Horrell but not the substance the supplier named as the possible source.

The company has announced a full investigation into how possible component contamination could have occurred after Estimate, owned by The Queen, also tested positive for morphine after her second in the Ascot Gold Cup.

Dodson & Horrell, horse and dog food manufacturer to The Queen since 2006, identified Alfalfa Oil Plus as the possible product which had been contaminated but said it may not have happened on its premises.

The company said it has launched a "thorough investigation" with an unnamed supplier of the product.

Dodson & Horrell believes contamination could have come from poppy seeds that occur naturally in the environment.

Estimate, trained in Newmarket by Sir Michael Stoute, produced one of the headline stories of last season with a nail-biting triumph in the Gold Cup at the Royal meeting.

She then went down narrowly to Leading Light in the defence of her crown last month.

The five-year-old mare is reportedly still in contention for next week's Goodwood Cup.

The British Horseracing Authority announced last week that five horses under the care of various trainers had shown the presence of morphine in their 'A' samples.

Trainers Charlie Hills and Gay Kelleway have also revealed horses in their care have tested positive for morphine.