Kauto Star is rated just 50-50 to run in the Cheltenham Gold Cup after a schooling fall.
Paul Nicholls had hoped he would brush the fall off but wanted to see more improvement by now and has had to declare his stable star's participation for the March 16 contest in doubt.
Regular work rider Clifford Baker suggested to Nicholls that Kauto Star was 50-50 to make the festival and owner Clive Smith agreed with the trainer to let the public know on Thursday.
Nicholls said in his column at www.betfair.com/paulnicholls: "There is no easy way to say this, but Kauto Star is only 50-50 to make the Cheltenham Gold Cup after a fall schooling.
"Kauto took a tumble when Ruby schooled him at the end of last week. I say tumble, but in truth it was a pretty awful fall.
"Clive was obviously consulted at all times and we immediately got Kauto checked over. Even though he trotted sound, it was plain to see that he was sore afterwards.
"He has had the best veterinary and physiotherapy care possible but we hoped that he would be showing more improvement by now.
"I spoke to Clifford after returning to the stables after a Cheltenham media day yesterday, and he said Kauto still wasn't fully recovered, remained quite stiff, and in his wise words he felt he was "only 50-50" to make the Festival at this stage.
"So I immediately spoke to Clive and we felt that it prudent that we put out this update today."
Kauto Star had been second-favourite for the Gold Cup behind last year's winner Long Run after beating the reigning champion twice this season.
Richard Horner, Head of Racing at Sky Bet, said: "Long Run is the clear 6-4 favourite from 2-1 after news broke of Kauto's schooling fall and the punters look sure to trust in last year's champion now after this disapointing news.
"Kauto Star has been pushed out to 4/1 from 3/1 and that price is likely to get bigger until we have confirmation of his participation."
Sunday Indo Living