Henrietta Knight and Philip Hobbs have both been fined �600 by the Jockey Club's disciplinary committee after each had a horse that failed dope tests.

Both animals tested positive for the prohibited substance acetylpromazine (ACP) earlier this year.

Knight's Hachty Boy failed a test after winning at Wetherby on 23 May while the Hobbs-trained Ashgar recorded a positive sample after his odds-on defeat at Plumpton on 30 March.

Neither trainer was able to explain how the substance had entered the bloodstreams of their charges.

No evidence of wrongdoing was uncovered by a subsequent investigation and both trainers accepted the minimum penalty under current Jockey Club guidelines.

Hachty Boy's positive result for ACP, a "stopping" drug, was especially surprising as the horse won his race by 11 lengths.

"Nobody has any idea how it came to be in the blood of that horse," said Knight.

"Like so many of these things, it will always remain a mystery.

"The rules say that as I am the trainer I have to be held responsible and that is what happened.

"Six hundred pounds is a lot of money and the whole affair has been a lot of grief."

Jockey Club public relations director John Maxse said: "In neither investigation was there any evidence suggesting an attempt to manipulate the outcome of the race.

"I can understand the trainers' frustration at not knowing the source of the ACP, but there are no plans to investigate either case any further."