Two racehorses trained by Barry Hills after he came out of retirement have been named among the eight which tested positive for morphine.
The British Horseracing Authority (BHA) said a disciplinary hearing would be held on 25 September.
But it believes the positive tests were caused by contaminated feed and that the trainers should escape sanctions.
It emerged last month that Estimate, owned by the Queen, was among those to give a positive post-race sample.
Hills, 77, retired as a trainer three years ago but was granted a temporary licence by the BHA following the death of his eldest son John in June.
He was the only one of six trainers in the morphine inquiry to remain anonymous until Wednesday.
The BHA said two of his horses tested positive for the banned substance - Soul Intent and Jolly Red Jeans after wins at Epsom and Chester respectively.
They are among seven cases which are sufficiently similar to be heard together with the others being Russian Realm, a Sir Michael Stoute stablemate of Estimate; Tony Carroll's Ocean Legend, Gay Kelleway's Royal Alcor and Strath Burn from Charlie Hills' stable.
The BHA is satisfied that the source of the positive samples was a contaminated batch of feed and that the trainers took all reasonable precautions to avoid breaching the rules of racing.
No financial penalty is likely to be imposed upon those trainers involved, but their horses are set to be disqualified from the relevant races, which means connections of Estimate would have to forfeit £80,625 in prize money for finishing second in the Gold Cup at Royal Ascot on 19 June.
The eighth case involves trainer Eve Johnson Houghton and her horse Charlie Wells, who tested positive after winning at Chepstow in June.
While that is also related to feed contamination, the BHA has said there are differences in the circumstances and this case will be heard separately in due course.
A product from Northamptonshire-based company Dodson & Horrell is suspected of being the source of the main contamination, although Johnson Houghton said she did not use the one in question.