Ireland last night lost the only gold medal it won at the 2004 Olympics in Athens when it was ruled that Waterford Crystal, the horse ridden by the showjumper Cian O'Connor, had been given banned substances.

This doping saga has been surrounded by controversy since part of the horse's B urine sample went missing last October from a laboratory at Newmarket when it was en route from France, following the announcement by the Federation Equestre Internationale (FEI) that the horse had tested positive for a banned substance.

The missing sample forced the second set of tests to be undertaken on a blood sample instead, which confirmed that minuscule amounts of Fluphenazine and Zuclophenthixol were present in the horse when he competed in Athens.

During an FEI hearing at Zurich airport that lasted 12 hours O'Connor had claimed the drugs - that are usually used as sedatives in humans - were given to Waterford Crystal by his vet for therapeutic reasons when he had been treating a minor fetlock injury.

The FEI's medical commission last year ruled that "these substances are being used in low dosages, to improve the manageability of 'hot horses' and are therefore regarded as serious attempts to influence the performance of the horse by medication".

After the decision O'Connor claimed that the disciplinary panel had exonerated him but, like most other sports, the FEI operates a strict liability policy that means the rider is solely responsible for drugs administered to his or her horse and they had no choice but to punish him.

"I am delighted the judicial committee has accepted and affirmed I was not involved in a deliberate attempt to affect the performance of my horse Waterford Crystal - as I have always maintained," he said.

O'Connor was among several witnesses who gave evidence at the hearing after arriving from taking second place at the first day of the Gold Tour competition in Arezzo, Italy, on ABC Landliebe, another of his horses which also failed a drugs test last year. Others who appeared on his behalf were his vet and groom.

"While I am disappointed that a technical infraction has resulted in the loss of the gold medal for Ireland, I wish to re-emphasise again today that neither I nor my vet James Sheeran has done anything wrong," said O'Connor.

"To get to the Olympics was my aim for many years and I have never even contemplated doing anything which might jeopardise this goal."

The committee disqualified the showjumper from the Athens Olympics, forcing him to forfeit his gold medal, which will now pass to Brazil's Rodrigo Pessoa. O'Connor was also ordered to contribute to the cost of the hearing and is suspended from the sport for three months.

He has 30 days to appeal against the committee's ruling at the Court of Arbitration for Sport, which he seems certain to do, arguing that a number of procedural errors arising from the theft of the sample means he should keep his medal.

• Sport