Video has emerged of Team GB's Charlotte Dujardin repeatedly whipping a horse during a training session.
Britain's joint-most decorated female Olympian pulled out of the Paris Games over the footage - recorded during a lesson at a private barn.
The 39-year-old said it was "completely out of character" and an "error of judgement" which "does not reflect how I train my horses or coach my pupils".
But she admitted "there is no excuse" and she's "deeply ashamed" and "sincerely sorry".
The dressage star, who has won six Olympic medals, including three golds, was due to compete in individual and team events in France.
Dujardin said the incident happened four years ago.
However, the whistleblower's lawyer told Sky News it was recorded two-and-a-half years ago while his client watched a lesson.
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Stephan Wensing said the woman had seen abuse several times from Dujardin.
"Charlotte Dujardin explained to the student she wanted to lift up the legs of the horse in the canter more - and then she used the whip as you saw on the footage," he said.
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The whistleblower didn't report it before as she was intimidated by the Briton's prestige, feared "victim blaming" and because people in UK dressage had warned her to stay quiet, said Mr Wensing.
He said his client felt she had to speak up with the Olympics imminent, but is afraid of reprisals.
"This year two more Olympic riders were suspended before Paris and she became a little brave," said the lawyer.
"Anyone can see whipping and beating like this is really tough and it causes pain," he added.
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Olympic and Paralympic dressage trainer Pammy Hutton has questioned the timing of the complaint, being on the cusp of the games.
She told Sky's Niall Paterson on the Sky News Daily Podcast: "The timing is suspect. It has to be suspect.
"If they really believed the validity of this video, why sit on it until the horses were leaving for France."
Lottery cash suspended
The International Federation for Equestrian Sports (FEI) has provisionally suspended Dujardin for six months.
UK Sport - the agency that gives out lottery money - said it was "disturbed" and has also removed her eligibility for funding pending a FEI investigation.
British Equestrian boss Jim Eyre said: "As the guardians of equestrian sport, we must uphold the highest standards of equine welfare - the horse's wellbeing is paramount.
"The allegations made are serious and the consequences far reaching."
In 2019, Dujardin was eliminated from the European Championships after a "very small amount of blood" was found on her horse's side.
The FEI said at the time it didn't imply any intent to injure the animal, but it broke a rule designed to protect the horses.
Dujardin said she was "devastated" by that incident and "nothing like this has ever happened to me before".
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Controversies before the Games
Dutch beach volleyball player Steven van de Velde is competing despite being sentenced to four years in 2016 for raping a 12-year-old.
The Netherlands Olympic Committee said he had "sought and received professional counselling" and "demonstrated to those around him – privately and professionally – self-insight and reflection".
Top tennis players including Aryna Sabalenka, Ons Jabeur and Emma Raducanu are boycotting the Olympics over concerns about the courts and injuries.
The competition is played on clay at the French Open, but players have normally transitioned to grass by now.
It's led to injury concerns over a lack of time to recover and adapt to the different surfaces.
Paris is the fourth Olympics where Russian athletes aren't allowed to compete under their own flag due to the Ukraine War.
Belarus athletes also can't represent their nation due to their government's support for Putin and must compete as 'neutral athletes'.
State officials from the countries won't be allowed to attend either.
French hurdler Wilfried Happio was accused of domestic violence in 2020 and sexual assault in 2022, being arrested for the latter - both charges were dismissed in court.
In July, further allegations were made and are being investigated - but he denies the claims and remains in the team.
Dujardin was born in Enfield in north London and started riding when she was two.
From a modest background but competing in an expensive sport, she bought her first Grand Prix horse in 2007 using money left to her by her grandmother.
Dressage involves the horse and rider performing artistic movements to music.
The current investigation comes more than two years after another Olympic champion Sir Mark Todd was suspended after a video showed him striking a horse with a branch.
Todd won two Olympic golds in 1984 and 1988 for New Zealand and was knighted in 2013.
