St Nicholas Abbey has suffered a serious setback in his recovery from a major leg operation.
The six-year-old had a bad bout of colic on Thursday for which he has had to have emergency surgery.
Coolmore tweeted: "Unfortunately St Nicholas Abbey had a major setback in his recovery during the night, suffering from a bad colic. He underwent emergency colic surgery at Fethard Equine Hospital this morning and the prognosis is very guarded at the moment."
On Wednesday, St Nicholas Abbey had complex surgery on a career-ending injury to a pastern.
The six-times Group One winner fractured a leg on the gallops at Aidan O'Brien's Ballydoyle stables on Tuesday morning in preparation for Saturday's King George VI And Queen Elizabeth Stakes at Ascot.
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 when he was beaten at the Curragh.
The six-year-old returned to something like his juvenile form when an easy scorer 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 two more times.
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 prize-money, having won nine of his 21 races.