The number of horses from the stables already caught up in the morphine controversy has risen to eight.

The initial stage of a British Horse Racing Authority inquiry has suggested contaminated feed as the source of the positives.

Seven horses had been reported to have tested positive for the banned substance, including Estimate, owned by the Queen.

"Further to the already reported seven samples testing positive for morphine, BHA can confirm an additional finding from within one of the yards that has already produced a positive," a BHA statement said.

"Once the outstanding 'B' sample analysis is complete, BHA will be able to confirm the identities of all the horses whose samples returned positive results."

Sir Michael Stoute confirmed Estimate and stablemate Russian Realm had shown traces of the drug, while Tony Carroll, Eve Johnson Houghton, Gay Kelleway and Charlie Hills have also said they train horses which had returned positives.

The BHA said that, with the exception of one case, the circumstances about which the positive samples were found were very similar and "point to the source of the positive test being contaminated horse feed".

Should the BHA disciplinary panel find that the source of the positives was contaminated feed, sanctions against those trainers involved will not occur - assuming they have "taken all reasonable precautions to safeguard against a positive test".

The horses will, however, be disqualified from the races in which post-race samples were taken.

That could mean that connections of Estimate, owned by the Queen, could have to forfeit STG80,625 ($A147,166) prize money for her second in the Gold Cup at Royal Ascot on June 19.