Star De Mohaison has been ruled out for the remainder of the season after sustaining a tendon injury.
The Paul Nicholls-trained chaser created a big impression in winning the Royal & SunAlliance Chase at the Cheltenham Festival last season and went on to record another impressive success at Aintree in April.
He returned to action with a win over timber at Prestbury Park last month before stretching his unbeaten run to five with a battling triumph at Sandown.
Barry Simpson, racing manager to owner Sir Robert Ogden said: "We were concerned over the horse's foreleg after the weekend and a scan today by Sir Robert's veterinary advisers has shown some damage to his near-fore tendon.
"The injury is a big disappointment for Sir Robert as he was very much looking forward to Star De Mohaison fulfilling his potential after winning his two races this term.
"We were hopeful he would develop into a Gold Cup horse given that he has run so well around Cheltenham in the past.
"However, it is very important the horse is given ample time to recover from the setback so he can hopefully return to training next autumn," he said.
Sponsors totesport reacted to the news by trimming Kauto Star into 2-1 favourite (from 9-4) for the Cheltenham Gold Cup in March, as well as cutting War Of Attrition to 3-1 (from 7-2).
Spokesman Damian Walker said: "This is another blow to ante-post punters after Kicking King and Celestial Gold were recently ruled out of the totesport Gold Cup.
Meanwhile, jockeys Brian Reilly and Dean Williams, both warned off for 18 months after a long-running inquiry into alleged corruption, are to appeal against the verdict.
It is understood the pair have been promised written reasons for the panel's decision before Christmas and will have the opportunity to launch their appeals up to seven days after receipt.
Williams said: "I am pretty disappointed with the way
things have gone, but hopefully my appeal will go well and I will get some time knocked off.
"I know that when you are warned off you are excluded from racing, but I'm hoping the HRA will allow me special dispensation for to continue working in a yard and ride out, because I have no other means of making a living."