Darlan, a leading contender for the Champion Hurdle, suffered a fatal fall at Doncaster on what should have been his final outing before the Cheltenham Festival.
Lining up in the Listed 32Red Hurdle as an even-money favourite, Darlan looked a likely winner as he cruised to the last in the hands of Tony McCoy. However, the Nicky Henderson-trained runner met the hurdle all wrong and crashed to the ground.
Although McCoy escaped unscathed, he stood himself down for the rest of the day.
Henderson, who was visibly upset by the loss of his up-and-coming star, said: "It's some game, isn't it? Why is it always the good ones?"
Owned by JP McManus, Darlan was among the top rank as a novice last term and had been expected to prove a major rival for ante-post favourite Hurricane Fly at Cheltenham next month.
The six-year-old graduated to Grade One success when running away with the Christmas Hurdle at Kempton on Boxing Day.
He is not the first star McManus has lost in tragic circumstances, with last year's Gold Cup winner Synchronised losing his life after a fall when running loose in last year's Grand National.
Darlan's death understandably overshadowed victory for reigning champion hurdler Rock On Ruby.
Countrywide Flame had set the early pace, with Rock On Ruby sat on his heels before making his challenge at the top of the straight under Noel Fehily. He was slightly in front and under pressure coming to the last when Darlan suffered his fall, leaving the 13-8 shot to come home three lengths clear of Countrywide Flame. Clerks Choice was last of the three finishers.
On a sad day for racing, the Tony Carroll-trained Mujamead lost his life in the 32RedPoker.com Handicap Chase, won by Be My Deputy, while at Wolverhampton on the same afternoon, Desert Vision also suffered a fatal injury when he fell three furlongs from home in an eight-and-a-half-furlong claimer.
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