Katie Walsh, who hopes to ride into the history books tomorrow by becoming the first woman to win the Grand National, endured a distressing start to the famous festival at Aintree when her horse died.
Battlefront, trained by her father, RTE pundit Ted Walsh, collapsed and died on the opening day of the three-day meeting in Liverpool.
It comes just days after the leading amateur rider became unwittingly embroiled in a controversy over her comments in defence of the Aintree Grand National's equine fatality rate.
"Very sad to lose Battlefront today. We had many great days and he was a great teacher. He was a gent and I will miss him very much!!" the jockey tweeted after the family's loss.
A veteran of 40 races, the 11-year-old horse, owned by her mother Helen, was in the front running for much of the early part of the John Smith's Fox Hunters' Chase, staged over more than a circuit of the Grand National course.
But the jockey pulled him up at the 11th fence. Initial reports said the horse, who has won 12 times under Walsh, may have suffered a heart attack.
Walsh attracted criticism from children's groups by suggesting during the week: "These horses are so well looked after. Better than some children, to be honest with you."
Last year, the Grand National came under scrutiny following the deaths two horses. Both horses had to be put down after suffering serious injuries while galloping loose without their riders.
Officials at Aintree reacted to criticism of the race by making a series of modifications to the controversial course, replacing solid timber stakes at the centre of fences with a plastic alternative.
A further five horses fell in the Fox Hunters' Chase yesterday, although none was significantly injured.
Andrew Tyler, director of Animal Aid, said: "The Aintree authorities and the British Horse Racing Authority have been claiming that major new safety measures and efficiencies would eliminate much of the risk associated with racing on the Grand National course.
"But today's Fox Hunters' Chase, in which Battlefront lost his life, was stomach-wrenchingly chaotic from start to finish Several horses fell or were pulled up, tired and potentially injured.
"It was both utterly depressing and served as confirmation that the Aintree authorities have got it badly wrong once again."
Battlefront was the 23rd horse to die on the Grand National course since 2000, Animal Aid said.
Aintree bosses made significant alterations to the course after last year's big race was marred by the death of According To Pete and Synchronised. That followed two fatalities in the 2011 race, Ornais and Dooney's Gate.
Old wooden fence frames were replaced with 'Easyfix plastic birch', dressed with spruce.
Aintree said the new fences were "kinder if the horse makes a mistake".
The height of the fences remains the same.
Animal rights group PETA criticised the course as "purposely punishing", and re-iterated its stance that "profit" was being allows trump concerns.
Ted and Katie Walsh team up once again with Seabass in the National tomorrow, having finished third in the race 12 months ago.
The only horse ahead of Seabass in the Grand National betting is On His Own, ridden by Walsh's brother, Ruby, for champion trainer Willie Mullins.