The mare would have been in pain because of the injury and should have received veterinary care, Durham Magistrates' Court heard.

Guest, 41, admitted one charge of causing unnecessary animal suffering by failing to exercise reasonable care and supervision.

He had already denied two other counts, in the prosecution brought by the Royal Society for the Prevention of Cruelty to Animals (RSPCA), which were formally dismissed by Deputy District Judge Professor Mary Hayes.

What's A Filly won at 100-1 in Kelso in December 2004, but was put down 12 months later.

She fell in a race at Uttoxeter in July last year and sustained an injury.

During her long recovery at Guest's yard, she developed an infection in her leg, which became too bad to treat.

The severity of her condition was only realised when the owners, unhappy at her slow recovery from the fall, moved her to stables near York.

Vets there discovered the infection was untreatable and she had to be put down.

The RSPCA began an investigation and charges followed.

Kevin Campbell, prosecuting, told the court that Guest, who won the 2001 Grand National on Red Marauder, should have overseen the horse's treatment.

"The allegation is put on the basis that he was in charge, it was his racing stable, and as such the buck stops with him," he said.

Head lad Richard Dalton, 41, and head girl Louise Tidman, 34, admitted omitting to provide veterinary care.

Mr Campbell said: "All three face an allegation of cruelty under section 1 of the Protection of Animals Act."

The defendants, all of Brancepeth Manor Farm, near Durham, will be sentenced this afternoon.

They entered their pleas on the first day of what was expected to be a week-long trial.