The brilliant seven-year-old, winner of six races at the highest level, including three Coronation Cups at Epsom, had to be put down after he suffered another bout of colic.
Murtagh was stable jockey at Ballydoyle when St Nicholas Abbey burst onto the scene as an unbeaten two-year-old – a campaign which included a breathtaking display in the Racing Post Trophy at Doncaster.
A disappointing effort in his sole three-year-old appearance in the 2000 Guineas at Newmarket was the last time Murtagh rode the horse, but the jockey expressed delight that he went on to show his true class.
He told Press Association Sport : “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.
“He was a top horse.”
Aidan O’Brien’s son of Montjeu faced a multitude of problems as he strived to recover from a fractured pastern he sustained last summer, when being prepared for the King George VI And Queen Elizabeth Stakes.
Just two days after he underwent a major leg operation at Fethard Equine Hospital last July, St Nicholas Abbey had to have emergency surgery to be treated for colic.
He then had a further setback last 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 laminitic changes in the left fore”.
Coolmore said St Nicholas Abbey will be buried at their graveyard in Co Tipperary.
A statement read: “Regretfully St Nicholas Abbey has lost his brave battle after suffering a bout of colic this morning.
“Surgery revealed a severe strangulating colon torsion that was unviable and he had to be euthanized 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.
“Coolmore would like to thank the surgeons, the international experts and all the staff at Fethard Equine Hospital who gave him such excellent care 24-7.
We would also like to thank the multitude of well-wishers for all the cards and messages of support for St Nicholas Abbey.”
St Nicholas Abbey built up a fantastic relationship with the trainer’s 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 worldwide stage in the 2011 Breeders’ Cup Turf and in the 2013 Dubai Sheema Classic at Meydan.
St Nicholas Abbey’s third Coronation triumph last June – his final racecourse appearance – took his career earnings to almost £5million.
Aidan O’Brien said at the time: “He’s an incredible horse and has everything you want in a racehorse – he quickens, he travels and is a joy to be around.
“Everyone has done a good job with him at home and he’s a privilege to be involved with.
“What everyone forgets is how much speed this lad showed as a two-year-old, he had that class all the time.
“He is an incredible horse.”
Epsom said it will always remember the exploits of St Nicholas Abbey.
The racecourse tweeted: “We’re all saddened to hear that St Nicholas Abbey has lost his battle today and will forever hold fond memories of his three Coronation Cup wins.”