Be My Royal has been disqualified from first place in the 2002 Hennessy Cognac Gold Cup by the Jockey Club.
David Casey and Be My Royal's win will be expunged from the records
The horse failed a post-race drugs test for morphine, the source of which is thought to have been a contaminated food supply.
Be My Royal's disqualification means original second Gingembre, trained by Lavinia Taylor, is promoted to first.
Trainer Willie Mullins, who attended the hearing, was not fined but was ordered to pay £5,000 in legal costs.
John Maxse, the Jockey Club's director of public relations, said: "It is unusual for an application of this type to be made by the Jockey Club.
"But on top of the cost of having counsel represent us, we have had to take extensive legal advice and additional work. So the order was considered appropriate in this case."
The full findings will be made available next week and Mullins has seven days from that day to appeal.
The Hennessy is the highest-profile race among a glut of positive tests for morphine at the time.
Between November 2002 and February 2003, 37 horses produced positive tests, of which 16 were winners.
So far, eight cases have been heard by the disciplinary panel.
All the horses have been disqualified and fines for trainers waived after the panel was satisfied in each case that the source of the substance was the same batch of feed.
Be My Royal was retired just a week after his Hennessy triumph when he suffered a serious injury in its next race.