St Nicholas Abbey will undergo surgery on a fractured pastern on Wednesday morning.
The six-times Group One winner suffered the injury during a routine gallop at Aidan O'Brien's Ballydoyle stable on Tuesday morning as he continued his preparations for Saturday's King George VI And Queen Elizabeth Diamond Stakes at Ascot.
Coolmore tweeted on Tuesday: "St Nicholas Abbey suffered a fractured pastern during a routine piece of work at Ballydoyle this morning. He is currently in a comfortable condition at Fethard Equine Hospital, where he is scheduled for surgery tomorrow morning."
Later on Tuesday evening Coolmore tweeted that the operation would be "long and complicated" and that fingers were crossed.
It read: "St Nicholas Abbey is scheduled for surgery under a highly experienced surgical team tomorrow morning at Fethard Equine Hospital. The operation to stabilise and repair will be long and complicated - we will need a bit of luck, so fingers crossed for St Nicholas Abbey."
A son of champion sire Montjeu, St Nicholas Abbey was champion two-year-old in 2009 when he crowned his season with a blistering display in the Racing Post Trophy at Doncaster. He was a red-hot favourite for the 2000 Guineas the following year, but could only finish sixth behind Makfi, and was not seen again until the following April.
He returned to something like his juvenile form when an easy winner of the Ormonde Stakes at Chester and went on to further Group One glory in the Coronation Cup at Epsom, a race he went on to win twice more.
Arguably his two greatest successes came on the world stage, however. He won the Breeders' Cup Turf in 2011, a victory which made his jockey Joseph O'Brien the youngest winner in Breeders' Cup history, and as recently as March he landed the Dubai Sheema Classic at Meydan.
St Nicholas Abbey amassed close to £5million in win and place prize money having won nine of his 21 races.