Warning - this article contains an image that some people may find distressing
Leading Irish racehorse trainer Gordon Elliott has apologised for a photo circulating on social media of him sitting on a dead horse.
Elliott said in a statement that he took a phone call and sat down on the horse "without thinking" and the image was taken "some time ago".
The Irish Horseracing Regulatory Board (IHRB) has begun an investigation.
The British Horseracing Authority (BHA) said it was "appalled" and "considering its own regulatory options".
"People who work in our industry believe their values - of caring for and respecting our horses - have been deeply undermined by this behaviour" the BHA said. " On their behalf, and on behalf of all horse-lovers, we say loudly that British horseracing finds this totally unacceptable."
Elliott, 43, who is based in County Meath, is a highly successful trainer who has won the Grand National three times, including twice with Tiger Roll.
"I apologise profoundly for any offence that this photo has caused," he said.
"I can categorically state that the welfare of each and every horse under my care is paramount and has been central to the success that we have enjoyed.
"The photo in question was taken some time ago and occurred after a horse had died of an apparent heart attack on the gallops.
"At what was a sad time, which it is when any horse under my care passes away, my initial reaction was to get the body removed from where it was positioned.
"I was standing over the horse waiting to help with the removal of the body, in the course of which, to my memory I received a call and, without thinking, I sat down to take it. Hearing a shout from one of my team, I gestured to wait until I was finished.
"Such background information may seem trivial at this time and will not allay the concerns of many people both within and outside the world of horse racing."
What has been the reaction?
Eight-time champion jump jockey Peter Scudamore said the photo "was an act of crass stupidity".
"It just hit the bottom of my stomach," said the 62-year-old, as he recalled his reaction to seeing the image.
"I think everybody in racing I know hoped it was fake, and then there was a slow realisation that it's not a fake. It's desperate sadness on so many fronts.
"It is just such an appalling image and I'm very sad about it."
Michael O'Leary has said Gigginstown, who own Tiger Roll and a number of other horses trained by Elliott, will "continue to support him and his team, as they work to recover from this deeply regrettable incident".
He says the photo is "unacceptable" and "grievous" but calls it a "momentary lapse of judgement", adding "we all make mistakes" and that Gigginstown accepts Elliott's apology.
Cheveley Park Stud, who own several horses trained by Elliott including the unbeaten Envoi Allen, say they are "truly horrified" by the photo but will not comment further until the investigation by the IHRB is over.
The Jockey Club, which owns Cheltenham and Aintree racecourses, said: "Clearly this is totally unacceptable and not reflective of the respect and care that racehorses receive from participants in our sport.
"The anger and upset across racing says it all. We understand the authorities are reviewing this as a matter of urgency."
Betfair said on Monday it had chosen to end its relationship with Elliott, who had been an ambassador for the betting company.
"While we recognise that Gordon deeply regrets and apologised unreservedly for his poor judgement, his actions are completely at odds with the values of the Betfair brand and that of our employees," a Betfair spokesperson told the BBC. "With that in mind, we have decided to discontinue our association with Gordon with immediate effect."
The charity World Horse Welfare said: "This photo looks abhorrent. We understand the trainer has apologised and there is an investigation ongoing."
Great British Racing, the promotional body of British racing, said: "Respect for our horses is at the heart of everything that we stand for in British racing and the shocking image is counter to that and betrays the work of thousands of people loving and caring for our horses on a daily basis - we condemn it in the strongest possible terms."
An IHRB spokesman said on Sunday: "The investigation is ongoing and will be dealt with as quickly as possible."
Horse Racing Ireland also condemned the picture and supported the IHRB investigation, adding: "From a disciplinary perspective, the matter is in process, so any further comment on the matter or the detail of the case at this time would not be appropriate."
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