A 15-strong group of judokas complained to the Japanese Olympic Committee (JOC) last month that they had been subjected to physical punishment by the team's head coach.

The group, which included athletes who took part in the London Olympics, says head coach Ryuji Sonoda routinely abused them, slapping them in the face and hitting them with thick wooden swords, like those used in the Japanese martial art of kendo. They also complained that some were forced to compete in matches while injured, reports said.

"We have asked the All Japan Judo Federation (AJJF) to investigate the case and improve their methods if the charges are true," a JOC official said.

AJJF head Koshi Onozawa said the federation had admonished Sonoda and other coaches, who had admitted several of the allegations.

"We received information that Mr Sonoda, the head coach of the female national team, might have been physically bullying athletes," Onozawa told a news conference. "Our executive office took this seriously and questioned both him and athletes, discovering the charges were largely true."

The AJJF told Sonoda and other coaches they must mend their ways and "will face a harsher punishment if a similar incident happens in the future", he added. Kyodo News said Sonoda did not deny the allegations when asked by reporters.