Fears of a new outbreak of "doping" hit British racing yesterday. The Jockey Club security department is investigating the circumstances surrounding the defeat of two short-priced jumpers, both of whom tested positive in post-race drugs tests.

There are worries that the pair may have been "stopped". One horse involved is Avanti, trained by Charlie Egerton, who was sent off at 5 to 4 when pulled up at Exeter in March. The other is Josh Gifford's Lively Knight, who started at 1 to 7 before his defeat in a three-runner race at Plumpton the same month.

Jockey Club director of public affairs David Pipe was unable to comment in detail on the cases as the investigations are still going on.

But he said: "We can confirm that the two horses did test positive. And we can confirm that a security department investigation is under way. They have been investigating since the horses tested positive, between two weeks and a month after the race.

"But the drug used will not released until after the investigation is completed and I can't confirm that the two cases are linked."

The last major doping cases to hit British racing came in 1990 when Bravefoot, Norwich and Flying Diva were found to have been administered Acetylpromazine.

Corkman Tom O'Connor partnered his first winner when guiding Karicleigh Boy to success in the opening Dene and Dyball Conditional Jockeys' Selling Handicap Hurdle at Exeter yesterday.

His mount was sent off 5 to 4 favourite and O'Connor justified punters' support with a confident ride, leading approaching the second-last flight and coming home four lengths clear of Coeur Battant.

"I came over here a year ago and it's a great thrill for me to ride my first winner. I'm feeling good and hope there's plenty more to come," said the 22-year-old. Lady Herries yesterday lost her appeal against the disqualification of Taufan's Melody at Ascot last month. The trainer went to the Jockey Club's disciplinary committee to contest the demotion of her charge from first place in the Tote Sunday Special Handicap due to accidental interference to Ridaiyma.

However, the committee upheld the view that the accidental interference had improved Taufan's Melody's placing.