British Olympic showjumping gold medallist Ben Maher is being sued for STG700,000 ($A1.28 million) amid allegations he made secret profits from horse deals.
Maher will defend an action in London's High Court brought by longstanding backers Mike and Emma Phillips.
They also envisage the sale of Tripple X, the horse he rode to team gold in London last year.
"It is all now in the hands of the court. It's a very sad end to a highly successful partnership," Emma Phillips told the BBC.
The 30-year-old Maher from Essex, east of London, has spent much of this year ranked as the world's number one rider.
The Phillips, who run the Quainton Stud in Buckinghamshire, southern England, said Maher acted as agent for them in the buying and selling of horses, but secretly made money on some deals by giving them incorrect prices.
He is alleged to have told the Phillips one of their horses, Tackeray, had been sold to a buyer in the United States for $500,000 rather than what they say is the actual sum of $850,000.
Maher has also been accused of making a secret profit or benefit on another five horses.
The Phillips told the BBC they had worked with the rider since 2005 when he was aged 22 and treated him "as one of the family".
Maher is said to have sold them a half share in his Olympic horse Tripple X.
But relations soured earlier this year and, as part of another civil legal action, the couple want to see Tripple X sold.
"We did all we could to provide the conditions that led to Ben's fantastic success at the Olympics last year and now feel broken-hearted," said Emma Phillips.
"We love the sport. We adore our horses -- but, in truth, we've become totally disillusioned."
Maher won team gold at London 2012 alongside Brash, Nick Skelton and Peter Charles.
