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Elliott, one of the sport's most celebrated figures in Britain and Ireland, apologized while confirming the authenticity of an undated photo that was shared across Twitter on Saturday, showing him sitting on a horse that had just died of a heart attack after a training run.
One of his most high-profile employers, Cheveley Park Stud, said it was "truly horrified" by the image and bookmaker Betfair dropped Elliott as an ambassador, saying his actions were not consistent with its "values."
They were sentiments echoed by the body that runs British horseracing in a scathing statement.
"People who work in our industry believe their values — of caring for and respecting our horses — have been deeply undermined by this behaviour," the British Horseracing Authority said.
In his convoluted explanation about an incident that he said occurred "some time ago," Elliott said he was waiting for the body of the horse — which has not been named — to be taken away when he received a phone call and sat down on the horse "without thinking."
The image shows him in a pose, holding two fingers out while sitting astride the horse.
The Irish Horseracing Regulatory Board is investigating the incident and the BHA said it is "considering its own regulatory options."
The Jockey Club, a commercial organization that owns many of Britain's top racecourses including Cheltenham and Epsom, said Elliott's actions were "not reflective of the respect and care that racehorses receive from participants in our sport."
"The anger and upset across racing says it all," it said.
Elliott has been a Grand National-winning trainer three times, first in 2007 with Silver Birch. He then trained Tiger Roll to become the first back-to-back winner of horse racing's most grueling jumps race — in 2018 and '19 — since Red Rum in the 1970s.
He has trained 32 winners at the Cheltenham Festival, where he has been the top trainer twice. He has finished runner-up in the trainers' championship in each of the last eight seasons and is again in second place this season just a few weeks before his array of stars — led by the unbeaten Envoi Allen — heads to Cheltenham.
Elliott did not come from a racing family like many leading trainers. He was a relatively successful amateur rider and then worked for Martin Pipe before striking out on his own.
Tiger Roll could yet make him the first trainer to win three consecutive Grand Nationals and the horse's owner, Michael O'Leary, is one of the few to come out in support of Elliott.
"We accept that this photograph was a grievous but momentary lapse of judgement by Gordon," said O'Leary, who runs the Gigginstown House Stud operation and is also CEO of budget airline Ryanair, "and not in keeping with our 15-year experience of his concern for and attention to the welfare of our horses.
"We all make mistakes, and what is important is that we learn from them and ensure we do not repeat them. We accept Gordon's sincere, profound and unreserved apology and we will continue to support him and his team."
The Irish Horseracing Regulatory Board has said its investigation will be dealt with "as quickly as possible."