Trainer Hughie Morrison has been charged with breaching the rules of racing after his horse Our Little Sister tested positive for banned substance nandrolone following a race at Wolverhampton in January, the British Horseracing Authority has confirmed.
The BHA said that, in accordance with its normal practice of assisting a trainer in circumstances such as these to establish the source of the administration, it has "carried out all reasonable inquiries it can with the full co-operation of Mr Morrison".
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Morrison - who could lose his licence for anything between a year and 10 years if found guilty - fiercely denies any wrongdoing, to the extent he has offered a cash reward for anyone that can help prove his innocence.
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.
The Group One-winning trainer told the Daily Telegraph: "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, 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.
"What on earth could I have to gain from this? Our Little Sister was a horse of limited ability, in a race with hardly any prize-money, and there was no unusual betting on it.
"Racing is my life. My reputation is everything. I might annoy a few people, but everyone knows my integrity is 100 per cent. I would never, ever do anything to besmirch the good name of the sport."
Our Little Sister raced once more, when down the field at Southwell on January 26, and has since been retired.
The Telegraph said Morrison's yard was raided at dawn on February 3, 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 January 2, a Bank Holiday Monday.
"She was left unattended for a significant time due to an injury to another of our runners.
"As the rules stand, the onus is on me as the licence holder to establish who administered an anabolic steroid to Our Little Sister.
"I have told the vast majority of my owners, who have been incredibly supportive and sympathetic.
"I employ 25 people directly, many of them living in the village, and it is causing enormous amount of distress to them as well as my family as I face the possibility of losing my licence and reputation."
Morrison said he intends to do his best to keep training winners until the matter is resolved.
He told At The Races: "We're doing our best to keep our heads down and training winners.
"You're basically committing suicide by giving any horse the substance which was found in My Little Sister back in January.
"We need to try and find the actual source and by offering a reward is our best chance.
"There are certain bits of information which we've requested from the BHA which I think would help us - at least in finding our way to the source - which they (BHA) have yet to give us. But I hope from now they might release this information.
"One got to the stage when we need more information. If we are going to prove ourselves innocent, we need to find the person, or party, who administrated this.
"I've discussed it with the police but they aren't interested unless I can get a witness."
The charges will now be considered by a disciplinary panel, and the BHA said that anyone with information in relation to the matter should contact the BHA by visiting www.britishhorseracing.com/racestraight/.
All information will be treated in the strictest confidence and the identity of any caller will remain anonymous.