St Nicholas Abbey has lost his six-month battle with ill health and injury.

A statement from owners Coolmore said the six-time Group One winner, who suffered a career-ending injury in July, was found to have inoperable colic and was euthanised.

"Regretfully St Nicholas Abbey has lost his brave battle after suffering a colic this morning," Coolmore said.

"Surgery revealed a severe strangulating colon torsion that was unviable and he had to be euthanised on humane grounds.

"This is extremely unfortunate as St Nicholas Abbey had been in terrific form, the laminitis was resolving very well and the fracture had healed better than expected.

"He will be buried in the graveyard here at Coolmore."

The horse suffered the leg injury in July and just two days after an operation on his leg, he had to have emergency surgery for colic.

He had a further setback in August when it was discovered a steel weight-bearing pin in a cannon bone had broken, after which he suffered another serious blow two months later when he developed mild laminitis.

Johnny Murtagh, stable jockey when St Nicholas Abbey began his career, said he was "potentially one of the best I've ever ridden".

"Unfortunately, things didn't happen for him as a three-year-old, but he has come back the last few years and shown everyone what a great horse he was," Murtagh said.

"He was a top horse."

St Nicholas Abbey built up a fantastic relationship with trainer Aidan O'Brien's jockey son Joseph in the latter part of his career.

O'Brien jnr rode him to two Coronation Cup triumphs and also secured notable victories on the world stage in the 2011 Breeders' Cup Turf and the 2013 Dubai Sheema Classic at Meydan.