Our Conor had to be put down following a fall in the Stan James Champion Hurdle at Cheltenham.
The brilliant winner of last year's Triumph Hurdle suffered serious injuries when taking an awful tumble for Danny Mullins and hampering The New One.
The horse was treated for back injuries for some time before vets took the decision to humanely put down the five-year-old.
Trained by Dessie Hughes, Our Conor was bought by Barry Connell for a reputed seven-figure sum after winning the Triumph Hurdle by 15 lengths 12 months ago.
The British Horseracing Authority tweeted: "Sad to confirm that Our Conor, after extensive treatment by veterinary surgeons, was humanely put down due to a suspected serious back injury."
Connell said: "It's is very sad, as he was such a young horse at the beginning of his career.
"He had already shown what enormous potential he had.
"I've not seen the fall yet, so I'm not exactly sure what happened.
"Danny said they went smart for the first few furlongs and then steadied it up.
"It is unusual for him, as jumping has been his forte and it was such a surprise to see him come down.
"The vets were very good, and I have to say that they gave him every chance, but there was nothing that could be done.
"It's unfortunate, but it goes with the territory - in this game one minute you can have a winner and the next one of your horses can die.
"They gave him sedatives straight away so he was in no pain, and they did everything possible to save him. Everything was top class.
"I've been in this game 40 years, and one of the reasons everyone loves it is that the highs are very high and the lows are very low.
"I've always admired him. He was only a baby and we knew that if he got placed this year we would be coming back next year with an even better chance."