Fellers, 63, pleaded guilty to engaging in illicit sexual conduct with a minor and two counts of second-degree sexual abuse in a federal courthouse in Portland, Oregon, local CBS affiliate station KOIN reported.

The ex-Olympic show jumper and coach is expected to be sentenced to four years in prison on the federal charges as well as a 30-month concurrent sentence on Washington County charges as part of a plea agreement.

Fellers, who was widely revered in the equestrian community, finished eighth on Flexible in the individual show jumping event at the London 2012 Olympics and was in the United States team that finished joint sixth.

He also won the International Equestrian Federation (FEI) World Cup Jumping Final in 2012 and 2015.

Fellers was arrested in 2021 for sexually abusing one of his students when she was 17 years old.

The victim, Maggie Kehring, has spoken publicly about the abuse and described Fellers as "a sort of father figure".

She moved to an apartment of her own at 15 to be near his barn and training centre in Oregon City.

She told Bloomberg last year that when she was 16, her coach suddenly expressed feelings for her and began grooming her until they had sex after her 17th birthday.

Their relationship reportedly continued until Fellers' wife, Shelley, caught them together at an Airbnb they were all sharing near a horse show in Michigan.

Fellers was subsequently ruled ineligible to participate in the sport after he and his wife were added to the US Center for SafeSport's list of suspensions, which was established by Congress in 2017 to investigate allegations of sexual misconduct in Olympic sports in the wake of the Larry Nassar abuse stories.

He initially denied allegations that he sexually abused a minor pleading not guilty to all four criminal charges.

Federal prosecutors brought charges in May 2023, the month after his wife Shelley filed for divorce.

He changed his plea to "guilty" in July 2023, as part of a plea deal.

Kehring told the Chronicle of Horse that many in the tight-knit equestrian community had criticised her once the ban on Fellers became known.

Her lawyer said those people owe her an apology especially now that Fellers has admitted to abusing her.

"For all the horrible people in the equestrian community that said terrible things about Maggie, I think there can be no clearer vindication for what she’s been through [than this]," Kehring’s attorney Russell Prince told the Chronicle of Horse.

"There’s quite a few people who owe Maggie Kehring and the Kehring family some heartfelt apologies."

Kehring herself told the Chronicle that she didn’t have further comment on Fellers' guilty plea, only that "the public record speaks for itself".

Following Fellers' arrest, Kehring and her family helped launch #WeRideTogether, a social media campaign to raise awareness around issues of sexual misconduct in equestrian sport.