Rocky Creek is to miss the Betfred Gold Cup at Cheltenham on Friday after being found to be suffering from ringworm.
The eight-year-old, runner-up in the Hennessy Gold Cup and Argento Chase in his two outings this season, will now be aimed at the Crabbie's Grand National at Aintree next month.
Trainer Paul Nicholls wrote in his Betfair blog: "The good news is that it is treatable in time to re-route him to the Grand National once again, and we will now prepare him for Aintree.
"I have had one or two horses that have suffered from ringworm this season - Ceasar Milan was one - and it is just a random problem that pops up occasionally.
"It is just incredibly disappointing that it has happened to a high-profile horse before a big race this time.
"We originally thought it was heat lumps but they got a lot worse and about a quarter of his body is now affected with it. It was confirmed as ringworm yesterday.
"It is highly contagious, so we have to be very careful, but we can manage that.
"Of course, it is hugely frustrating as I thought he was going to Cheltenham with a live outside chance in the Gold Cup, but it is not an injury as such and we still have a big target to aim him at this season."
"It is highly contagious, so we have to be very careful"
Meanwhile, the Willie Mullins-trained On His Own is expected to be supplemented for the Gold Cup at a cost of £27,500 later this morning.
The Graham Wylie-owned 10-year-old has won the Thyestes Chase and Bobbyjo Chase this term, and such was the manner in which he claimed the latter that Mullins admitted afterwards that "he's just going out of handicaps and could win a really good race".