Trainer Gordon Elliott has apologised 'profoundly' for an image that has gone viral on social media of him sitting on a dead horse.
The Irish Horseracing Regulatory Board (IHRB) is 'appalled' by the image that was 'taken some time ago' and will investigate it.
The BHA said: "The BHA is appalled by the image that appeared this weekend. We expect all those in our sport to demonstrate respect for horses, on the racecourse, in the training yard, on the gallops, and wherever they have horses in their care.
"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, a three-time Grand National winner, including twice with Tiger Roll, is a leading name in the sport based in County Meath. His other triumph at Aintree was with Silver Birch in 2007.
"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."
An IHRB spokesman said: "The investigation is under way, and it will be dealt with as quickly as possible."