Wrestler Hakurozan and Roho

Russian sumo wrestler Hakurozan and his brother Roho at the Nagoya Grand Sumo Tournament in Nagoya. Photograph: Kyodo News/AP

The head of Japan's embattled national sport of sumo faced calls to resign today after two wrestlers, including one of his proteges, tested positive for marijuana.

The Japan sumo association said urine tests conducted at several stables yesterday revealed marijuana use by the Russian siblings Roho and Hakurozan.

The revelation of drug use among sportsmen known for their Spartan training methods and supposedly disciplined lifestyle is a huge embarrassment for the sumo authorities.

It comes a fortnight after another Russian, Wakanoho, became the first sumo wrestler to be expelled in the sport's 2,000-year-history. Police found a marijuana cigarette in his wallet, which had been handed in after he dropped it on the street.

Wakanoho, 20, was arrested after admitting he had bought a small quantity of the drug in Roppongi, a notorious entertainment district in Tokyo. A pipe used for smoking cannabis was found in his apartment.

Today's revelations managed to knock the political fallout from prime minister Yasuo Fukuda's sudden resignation off the top of the TV news agenda.

The sumo association said it had carried out surprise tests on all 69 wrestlers in sumo's top two divisions. Only Roho, 28, whose real name the association gave as Boradzov Soslan Feliksovich, and Hakurozan, 26, tested positive, officials said.

"It is possible that they inhaled very recently, probably within the last two days," said Shohei Onishi, a sumo anti-doping official.

Hakurozan, whose real name is Baradzov Batraz Feliksovich, is coached by Kitanoumi, a retired grand champion who now heads the sumo association.

Both wrestlers denied smoking marijuana. "I have never used or even touched the stuff," Roho told reporters. "I want another test to be conducted at a hospital I can trust."

His younger brother said he was confident that further tests would clear his name. "I don't mind of they investigate me or test me," Hakurozan said. "I'm confident that I definitely won't test positive."

Although possession of marijuana is punishable by up to five years in prison, Japanese law carries no penalty for simply smoking it.

Sumo authorities are under mounting pressure to show zero tolerance towards drug use as it battles to salvage its already tarnished reputation.

The ancient sport was rocked earlier this year by the arrest of Junichi Yamamoto, a stable master, on assault charges following accusations that he had ordered the beating by three of his wrestlers of a 17-year-old trainee in June last year. The victim collapsed and died the following day.

Sumo elders have also had to fend off accusations of match fixing and have been ordered to clamp down on the widespread physical abuse of younger wrestlers.

Last year the association banned Asashoryu, one of two reigning "yokozuna" - grand champions - for two tournaments after he was filmed playing in a charity football match in his native Mongolia, despite pulling out of a goodwill sumo claiming to be injured.

Even so, sumo authorities will find it hard to resist charges that it singles out foreign wrestlers for harsh punishments for what many consider minor misdemeanours.

While Asashoryu was forced to sit on the sidelines for three months, Toki, a Japanese wrestler who struck and killed a pedestrian while driving in Osaka in 2000, was banned for just one tournament.

Police said they would examine further test samples from Roho and Hakurozan. The results are expected within the next few days. A second positive result would make it almost impossible for Kitanoumi to stay in his post.

Mitsuru Yaku, a fellow sumo association member, told a TV interviewer that it was "natural" that Kitanoumi would be sacked, adding that he believed more damaging revelations were on the way. "I don't think for a moment that all of the puss has come out," he said.

Kitanoumi, 55, still holds the record as the youngest wrestler to attain the vaunted yokozuna status, at the age of 21 years and two months.