Go Native suffered a fatal injury when breaking a leg on the gallops on Friday morning, said trainer Noel Meade.
The nine-year-old was a triple Grade One winner and had looked back to his best this season following a long absence.
Meade said: "We're all in a state of shock at the moment. He was just doing a very steady canter. I was stood about 100 yards away from him and all of a sudden I heard this almighty crack and he'd somehow broken his leg above the hock."
He added: "I don't know why these things happen. He was the first one to come up the straight gallop after it had been harrowed. We are all completely devastated. It's a cruel blow for his owners and everyone in the yard."
Owned by the Docado syndicate, Go Native returned from an absence of 939 days to be third on the Flat at Navan in October and went a couple of places better subsequently at Galway.
He went back over hurdles in the Morgiana Hurdle at Punchestown last weekend only to fall at the final flight when a length behind the winner, Hurricane Fly.
Referring to the Morgiana, Meade said: "You have to question whether he did something in the fall, but although he's not done any fast work, he's been hacking all week and has seemed fine.
"We had the physio check him out thoroughly earlier in the week and she was happy that he was perfect. We'll never know now. He had all that time off and came back as good as he ever was and for this to happen is just incredible.
"You have to be a hard man in a game like this, I tell you. I know there are lots of other businesses where you get knocks, but there can't be many harder games than this."
19 June 2013
Perfect Annecdote at AscotGodolphin swoop for Libertarian
Doyle hits treble top on Rizeena
Doyle fits the bill in Hunt Cup
Toronado pointed towards Goodwood
O'Brien keeps faith in Camelot
Kazeem crowned Prince of Ascot
Cambridge Dun and dusted
Perfect Ten in Jersey
Plenty of rain needed for Approval
Banoffee pencilled in for Haydock
Classic rematch in Coronation
Guest has excellent chance
Brittain sweet on Rizeena
Team O'Brien hope for perfect Ten
Chigun aiming for Cambridge glory
Camelot bids for redemption
O'Neill ace proves too sharp
Ballydoyle in command in Coventry
Dawn delivers in thriller
Sole powers to sprint glory
O'Brien sounds War cry
18 June 2013
Stevens is king of the CastleAl Thani snaps up Sandiva
Minute's silence remembers Cecil
Duntle on Cambridge duty
Alive and Just in Ribblesdale mix
Passage heads Gold field
Deegan considers Soprano options
Toro out for Palace glory
Cecil's words not lost on Ponsonby
Dawn out for Palace redemption
Hawkwood supplemented for Hardwicke
Flotilla To Drop Back In Trip
Cecil's Chigun in Cambridge action
Al Kazeem and Camelot clash again
De Kock hoping to make history
War ready for Royal Ascot battle
Kingdom set to reign over Ascot
17 June 2013
Magician set for Palace showdownCat clings on for Carroll
Three more winners for Lady Cecil
Monaghan delivers on Midnight
Treve in stunning Diane win
Sapphire target for Trees
Beckett mulls Secret options
Nicholls has broken wrist
16 June 2013
Songbird aiming to fly highMovie has Newmarket on radar
Fahey juveniles primed for action
Stubbs leads O'Brien Coventry team
Salsify on summer break
Kempton target for Benbulben
Top trio head Palace runners
Shea Shea shoots for Stand glory
Animal heads Queen Anne field
Saint-Cloud aim for Dunaden
Magician to be declared for Palace
Soul all heart in York sprint
Kingsgate rules Musselburgh
Sam too sharp for rivals
Hughes makes it pay with Knight
Balding on mark with double
Morawij in a hurry in Scurry
Bartley is queen for a day
Big guns to clash in Coronation