Sprinter Sacre will not defend his Queen Mother Champion Chase crown at Cheltenham next month.
The Nicky Henderson-trained two-mile king was sensationally pulled up at Kempton on December 27, where he was quickly diagnosed as suffering from an irregular heartbeat.
That problem righted itself, and he has been closely monitored since by equine specialist Celia Marr, steadily pleasing his team and with a racecourse gallop in the offing.
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 Cheltenham and the rest of the season.
Henderson told Press Association Sport: "He worked (on Saturday) as normal, but I just needed to see the old Sprinter back yesterday. Everything had been going very well, his work was building up. It had been OK, but I was looking for that something he shows, he is extraordinary and always used to give us that at home.
"I talked extensively with Caroline and Raymond Mould, who have been very understanding.
"I said the other week it was 50-50 (getting to Cheltenham) but quietly I thought it was above that, I honestly thought we were going the right way.
"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 yesterday. It's not what he's made of."
Henderson went on: "He worked with Triolo D'Alene, Nico (De Boinville) rode him as he has every day for the last three years, and he had his heart monitor on.
"Triolo is a good work horse and anything that works with him has to be good, he is the one horse I use as a lead horse for Sprinter Sacre as he needs a good gallop on the way to the Grand National and wherever he goes before that.
"The old Sprinter Sacre would have found it very easy - if a normal horse works well with Triolo you'd say 'get on that next time' but Sprinter in fairness is unique, it's as simple as that.
"Simonsig used to be his workmate last year but we lost him (to injury). He will be back and I'm confident Sprinter Sacre will be back, too.
"But I can't ask him to go to a racecourse next weekend. We all watch him day in day out, his blood is good and his scopes, and Celia says his heart is good. It's not his heart. He's just missing something, something like 10 per cent, but that is crucial when you are in the zone we are in.
"I don't feel I want to gallop him on a racecourse next week, he needs a racecourse gallop (to get to Cheltenham) and I can't ask him to do it. He is 100 per cent sound, he's above 90 per cent but you need to be 100 per cent, that's where you need to be. He needed to go to a racecourse.
"We talked for a long time last night and again today. We've come up with our decision and that is that, the Moulds have been very understanding. It just shows all the ups and downs.
"Last year was phenomenal, particularly with Sprinter. He took us to places no horse has done for years.
"I can't thank Celia and everybody at home enough.
"At least he is sound and his heart is good, therefore there must be every chance he will be back next season 100 per cent and let's hope the old Sprinter is back in the Tingle Creek in December, that's what matters to us.
"Look out everybody because believe me we will get him there."
Paddy Power make the Gary Moore-trained Sire De Grugy, twice a Grade One winner this season, the 5-4 favourite for the BetVictor-sponsored Queen Mother on March 12.