Hughie Morrison could lose his training licence after one of his horses tested positive for an anabolic steroid, the Berkshire handler told the Daily Telegraph.
Morrison is facing the prospect of a suspension of between one and 10 years after he said Our Little Sister was found to have contained nandrolone in her system after a race at Wolverhampton on 14 January.
Our Little Sister finished last of eight runners in an extended two-mile handicap, in which she was sent off at odds of 12-1.
Morrison has denied all wrongdoing and said he has offered a cash reward for information that would lead to his name being cleared.
The Group One-winning trainer said: “I am doing everything I possibly can to get to the bottom of this. I’ve reported it to Thames Valley Police who, along with the BHA [British Horseracing Authority], were supportive in my offering a reward to see if this could take things forward.
“We have brought in one of the world’s leading experts from America. We’ve found out through our own research that it cannot be a contamination issue with this particular drug and we have all but ruled out the slim chance she was producing it herself.
“We have carried out an extensive search around the stable. We questioned a possible flaw in the testing but a B sample sent to Paris also came back positive, along with our own hair sample.”
Our Little Sister raced once more, when down the field at Southwell on 26 January, and has since been retired.
Morrison’s yard was reportedly raided at dawn on 3 February, when blood samples taken from all 77 horses, including Our Little Sister, returned negative results.
Morrison said: “She was away from the yard during the period that we suspect the substance was administered, when she was one of three runners we had at Southwell on 2 January, a Bank Holiday Monday. “She was left unattended for a significant time due to an injury to another of our runners.”
A BHA spokesman said in a statement it was “not policy to comment on ongoing investigations”.