Cheltenham Gold Cup day began in the worst possible fashion when Sir Erec hot favourite for the JCB Triumph Hurdle, suffered a fatal injury.

Trained by Joseph O’Brien, Sir Erec was a top-class horse on the Flat for O’Brien’s father, Aidan, finishing third to Stradivarius at Ascot on Champions Day.

A winner of both his starts over hurdles, the four-year-old was still an entire and 11-10 to remain unbeaten over obstacles, despite suffering a stone bruise in the week before the race.

He spread a plate on the way to the start, but stood perfectly motionless and calm while the farrier fitted a new one — causing a delay to the race.

The first mile was going to plan for jockey Mark Walsh, but on jumping the fourth hurdle, Sir Erec broke down badly and had to be euthanised.

David Sykes, director of equine welfare at the British Horseracing Authority, felt Sir Erec’s injury could not possibly have been predicted.

He told ITV Racing: “Sometimes there are catastrophic injuries. This morning he was examined by a veterinary surgeon and trotted up and he said he moved very well.

“There was no indication of lameness, there was no indication of injuries.

“Whether he made a mistake at that hurdle or not and landed awkwardly — there was no reason we could predict that injury to that horse today.

“He was shod and the veterinary surgeon trotted him up in the same way. The shoe was having no effect. Putting a shoe on wasn’t going to cause that sort of injury.

“It’s one of those catastrophic injuries that occur unexpectedly. We are not able to predict those.”

A statement from Cheltenham said: “Sadly Sir Erec sustained an injury in the JCB Triumph Hurdle.

“He was immediately attended to by the veterinary team, who very sadly determined that it was necessary to euthanise him on welfare grounds.

“As animal lovers, we hate to lose any horse and of course shall review this incident.

“Our thoughts are with Sir Erec’s connections.”