Roaring Lion has undergone surgery for colic in New Zealand where he was due to stand at Cambridge Stud this season.
Henry Plumptre, chief executive of Cambridge Stud, said the horse had been admitted to Cambridge Equine Veterinary Hospital early on Saturday morning, just 15 minutes after leaving quarantine where he had been since he arrived from England.
Roaring Lion underwent emergency surgery and on Sunday was reported to be doing well.
"We want to act in the best interests of Sheikh Fahad and our shareholders," Plumptre said.
"Following major surgery, we feel it is appropriate to withdraw the horse from service with all shareholders being fully refunded.
"Our best-case scenario is that Roaring Lion makes a full recovery and can be returned safely to the UK.
"While everyone at Cambridge Stud is shattered, we feel the obligation to Sheikh Fahad, David Redvers and our shareholders is important. It is a massive blow to lose Roaring Lion like this, but his ongoing welfare is now our prime concern."
Europe's Horse of the Year for 2018, Roaring Lion is the winner of four Group One races.
He will be closely monitored over the next few days. - AAP