Sumo wrestling's greatest champion has been demoted as a trainer after an investigation found one of his students repeatedly assaulted other wrestlers.

Hakuho Sho, 38, was downgraded to the lowest rank for sumo elders and forced to take a pay cut.

His protege, Hokuseiho Osamu, admitted to slapping the faces of subordinates and hitting them with a broom handle.

Hakuho has apologised, saying he takes responsibility for being unable to protect the victims.

The Japan Sumo Association carried out the probe after a tip-off on social media in January, the country's public broadcaster NHK reported.

The Japan Times said that Hokuseiho, 22, also slapped junior wrestlers' backs and testicles, and ignited insecticide spray towards them.

"I deeply regret having used violence against my stablemates," Hokuseiho said, as quoted by the Times.

The Mongolian-born wrestler had a successful career, winning his first 21 matches. He pulled out of sumo tournaments earlier this year over a knee injury - around the same time as the allegations against him started to surface, Japanese media reports.

The sumo association said it had accepted Hokuseiho's offer to retire, local media reported.

Also Mongolian-born, Hakuho won a record number of sumo titles in his career and retired in 2021.

A year later, he took command of his sumo stable, where young wrestlers are trained, and became known as stablemaster Miyagino.

Demotion in rank is considered the third most severe punishment for sumo elders after dismissal and a recommendation for retirement. (BBC)

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