Kauto Star is only "50-50" to make the Betfred Cheltenham Gold Cup after a fall when schooling last week, according to trainer Paul Nicholls.
Nicholls said the dual Gold Cup winner took a nasty tumble at his Ditcheat yard under Ruby Walsh.
Speaking in his Betfair blog, Nicholls said: "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."
Nicholls said: "As a trainer, it is all too easy to sit on information when it comes to your horses.
"They have so many minor worries, niggles and setbacks - most of which turn out to be nothing of long-term importance - that you could be constantly updating on each of your string, to the press and the public, on a hourly, daily and weekly basis.
"But our policy at Ditcheat is always to be up front and honest about the wellbeing of our horses - such as the coughing scare last week - and especially so when it is a public, and once-in-a-lifetime horse, such as Kauto.
"In my opinion, the public have a right to know. In many ways, he is as much theirs, as Clive Smith's and ours.
"So I am sad to report that his participation at the Cheltenham Festival is in doubt.
"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 he would be showing more improvement by now.
"I spoke to Clifford (Baker, head lad) 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 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 and Nicholls added: "Next week is crucial for the horse.
"He is still cantering each day but I can assure you right now that Kauto Star won't be going anywhere near Cheltenham unless we are all convinced - Clive Smith, myself, Clifford and the team here - that he is 110% right.
"Even if there is the slightest doubt in our minds, then he stays at home. No question. We will do what is best for the horse. Always have, always will. End of story.
"But, along with Desert Orchid, I suppose if ever a horse has captured the public's love and imagination it is Kauto Star. We saw that when he won the Betfair Chase at Haydock.
"It even had this old cynic bursting with pride for the horse, fighting back the tears on my proudest and most emotional day as a trainer.
"So, of course, it will be devastating for all concerned if Kauto Star wasn't able up to line up on March 16 and try to win a third Gold Cup. Because he has been in the form of his life this season.
"But one thing I think we have all learned with Kauto is never to write him off.
"Yes, having this setback so close to the Festival is clearly a major concern, but don't confuse Kauto's kind and inquisitive nature with softness.
"He is a hard so-and-so too - you learn to be when your best mate is Denman - and it wouldn't surprise me in the slightest if he was back firing on all cylinders at home before Cheltenham, and went on to beat Long Run for a third time this season.
"But at this stage his participation does hang in the balance."