Sprinter Sacre will not defend his Queen Mother Champion Chase crown at the Cheltenham Festival next month, Sire De Grugy the new favourite.
The Nicky Henderson-trained two-mile king was sensationally pulled up at Kempton on December 27 and quickly diagnosed as having an irregular heartbeat.
That problem righted itself and he has been closely monitored since.
However, the eight-year-old did not show his usual brilliance in a piece of work at Henderson's Seven Barrows base on Saturday, and the decision has been taken to miss the rest of the season.
Henderson said: "He worked as normal but I just needed to see the old Sprinter back.
"Everything had been going well, his work was building up. It had been okay, but I was looking for that something he shows, he is extraordinary and always used to give us that at home.
"It's just not all there and I can't ask a horse of his standing, I can't sacrifice him, I think it is fairer to everybody – the horse told us what he wanted to tell us."
Paddy Power make Sire De Grugy 5-4 favourite for the Queen Mother on March 12.