British dressage star Charlotte Dujardin withdrew from the Paris Olympics on Tuesday after a video emerged allegedly showing her mistreating a horse.
Dujardin, a three-time Olympic champion and the joint most-decorated British woman Olympian, said she had made an "error of judgement" and expressed deep remorse.
The International Federation for Equestrian Sports (FEI) suspended her with immediate effect, just days before the start of the Games.
The governing body said it had received a video "depicting Ms Dujardin engaging in conduct contrary to the principles of horse welfare".
"This video was submitted to the FEI by a lawyer representing an undisclosed complainant," the FEI added.
"According to the information received, the footage was allegedly taken several years ago during a training session conducted by Ms Dujardin at a private stable."
The FEI said Dujardin had confirmed she was depicted in the video and had acknowledged that her conduct was "inappropriate".
It said she had asked to be provisionally suspended pending the outcome of the investigations and voluntarily withdrew from the Paris Olympics.
"The FEI condemns any conduct contrary to the welfare of horses and has robust rules in place to address such behaviour," it said.
"Our commitment to ensuring the highest standards of equine welfare and sportsmanship remains unwavering."
Earlier, Dujardin said a video had emerged from four years ago showing "an error of judgement during a coaching session.
"Understandably, the International Federation for Equestrian Sports (FEI) is investigating and I have made the decision to withdraw from all competition -- including the Paris Olympics -- while this process takes place," she said.
"What happened was completely out of character and does not reflect how I train my horses or coach my pupils. However, there is no excuse. I am deeply ashamed and should have set a better example in that moment."
FEI president Ingmar De Vos said: "We are deeply disappointed with this case, especially as we approach the Olympic Games in Paris.
"However, it is our responsibility and crucial that we address any instances of abuse, as equine welfare cannot be compromised.
"Charlotte has expressed genuine remorse for her actions, and we recognise and appreciate her willingness to take responsibility."
Dujardin was eliminated from the European Championships in 2019 after blood was found on her horse, Mount St John Freestyle, in a post-competition check.
The FEI said the action taken then did not imply there was any intent to injure the horse.
Dujardin, 39, could have become Britain's most decorated woman Olympian in Paris. A medal of any colour would have taken her clear of Laura Kenny, with whom she is currently tied on six medals.
She was due to compete in both the individual dressage and the team event alongside Carl Hester and world champion Lottie Fry at the Paris Games.
She is expected to be replaced in the team by Becky Moody.
Dujardin won individual and team golds at London 2012 while riding Valegro, with the pair retaining the individual title and winning silver in the team event at Rio 2016.
Olympic and equestrian authorities have taken an increasingly strict line against mistreatment of animals in recent years.
During the delayed Tokyo Games in 2021, a German modern pentathlon coach was thrown out for striking a horse.
Authorities launched a review of the sport, resulting in the equestrian element being removed and replaced with an obstacle course in Los Angeles in 2028.
The dressage competition at Paris 2024 begins on July 30 at the Chateau de Versailles.
