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 added: "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 well-being 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 that 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 that it prudent that we put out this update today.
"Next week is crucial for the horse."