The British Horseracing Authority says its inquiry into trainer Stephen McConville is ongoing after one of his horses was ordered to be withdrawn from the Cheltenham Festival in March.
McConville is under investigation after banned substances were allegedly found in the stable of Anseanachai Cliste, reported the Times.
A BHA spokesman said: "We cannot comment on an ongoing investigation."
McConville has not responded to requests to comment on the case.
Cheltenham stewards ordered the horse, trained in County Armagh, Northern Ireland, to be withdrawn from the Foxhunter Chase on 17 March.
"They could not be satisfied that the horse had been administered only normal feed and water on race day," said a stewards' report.
"They interviewed the owner, the trainer, the veterinary officer and the equine welfare integrity officer. Having heard their evidence, the stewards ordered the horse to be tested and referred the matter to the head office of the British Horseracing Authority."
Anseanachai Cliste won the Ulster Grand National at Downpatrick nine days later, after which the Irish Turf Club says a drugs test came back negative.
Asked about the case, McConville said after the Downpatrick win: "I wouldn't like to talk about Cheltenham. We'll see after the inquiry."