Sumo, Japan's 2000-year-old national sport, has begun to feel the strain amid allegations of bout-fixing, tax evasion and sexual impropriety.

In the ring, or dohyo, sumos compete according to strict code of conduct.

The dohyo itself is considered sacred ground that is blessed by a Shinto priest before every bout and purified by the stamping of the wrestlers and the throwing of salt.

But a spate of sordid stories in the Japanese tabloids has revealed that there is a big difference between the sport's squeaky-clean image and reality.

The latest scandal centres around former sumo Keisuke Itai, who claimed that fixed bouts are rife. Now a restaurant owner, he has admitted that as a komosubi - the fourth highest sumo rank - he also threw matches.

"I regret what I did in the past," he said.

"Sumo wrestlers are true athletes, so there should be no fixed matches."

Sumo officials have hit back, insisting that there is no problem. Nonetheless, Itai has become something of a celebrity and continues to draw unwelcome attention to a sport that has only just recovered from the last time one of its number spoke out.

On that occasion, in 1996, a wrestler told of match-fixing, sexual scandals and tax cheating in a series of shocking newspaper articles.

In the immediate aftermath, one of Japan's top sumo families was handed a massive bill for back-taxes to the tune of ¥400m ($3.7m).

The perceived problems with the sport even extend to the distinctive physiques of the sumos themselves.

Fans have begun to complain that the wrestlers lack "athleticism" - underlined by the number of competitors who have suffered injuries largely due to their excessive weight.

Here, the sumo federation has acted. Wrestlers were required to submit to body fat measurments and then told to go on a diet.

Some enthusiasts blame weight problems on the old system of training in "stables", where rigorous physical sessions are interspersed with heavy eating to help the sumos gain bulk.

Others point to the emergence of a new breed of wrestler, who joins the sumo circuit directly from college clubs and is never subjected to the stringent routine of a stable.

Whatever the issues surrounding over-large sumos, many commentators have declared themselves unconcerned by the latest furore over bout-rigging.

Andy Adams, the publisher of Tokyo-based Sumo World, said the practice of throwing matches was not new and went back hundreds of years.

"There's an old tradition that if you rub my back this time, next time ... I'll rub your back," said Adams.

"It's unspoken. Nobody says anything to anybody. It's just sort of understood."