TOKYO: Japan's ancient sport of sumo suffered another humiliating blow Wednesday when grand champion Harumafuji retired after a brutal assault on a rival wrestler while out drinking, his gym boss said.

Harumafuji's stable master Isegahama told local media the Mongolian "yokozuna" had caused embarrassment to the Japan Sumo Association (JSA) over an incident last month that left countryman Takanoiwa with a fractured skull.

His exit mirrors that of another top-ranked Mongolian, Asashoryu, who promptly stepped down in 2010 after being accused of breaking a man's nose in a drunken brawl outside a Tokyo nightclub.

Japanese Prime Minister Shinzo Abe weighed in on the debate, while news of Harumafuji's demise even nudged North Korea's latest missile launch off top spot on many news programmes on Wednesday morning.

Yokozuna are expected to be beyond moral reproach but the writing was on the wall for Harumafuji after he confessed to hitting Takanoiwa for texting his girlfriend while he was scolding him over his poor attitude.

The 33-year-old Harumafuji, who reached sumo's hallowed rank five years ago and whose real name is Davaanyam Byambadorj, denied reports he had used a beer bottle in the attack but admitted punching Takanoiwa and bashing him with a karaoke remote control.

Takanoiwa, 27, was hospitalised after suffering concussion and a fractured skull base in the brawl, which broke out at a bar in the western Japanese city of Tottori during a regional sumo tour in late October, according to local media.

His stable master subsequently reported the incident to the police, who invited Harumafuji in for questioning.

Prime Minister Abe, who was busy dealing with Pyongyang's missile test, expressed disappointment over sumo's latest scandal during a scheduled parliament session.

"This incident of violence in the world of sumo, while still under investigation by the Japan Sumo Association, is extremely regrettable," he told the upper house.

"First of all it is important to clarify the facts quickly, then the sports ministry will take appropriate measures."

The PM's words were echoed by Yoshimasa Hayashi, the education and sports minister.

"We have to absolutely eradicate violence in sport," he said.

"People involved in sumo must be aware that sumo is the oldest sport in Japan. They must act responsibly to prevent a repeat of this violence and never betray Japanese people's expectations."--AFP