TOKYO, July 27 (Reuters) - Japan's ancient sport of sumo reacted with dismay on Friday to news that firebrand "yokozuna" Asashoryu secretly took part in a soccer match.

The 150-kilogram Mongolian provoked anger by playing alongside former Japan international Hidetoshi Nakata in his home country after pulling out of a regional sumo tour with a back injury.

"He is making a mockery of Japanese culture!" Japan Sumo Association (JSA) disciplinary committee member Masako Uchidate was quoted as saying in Friday's Sports Hochi daily.

"If he's well enough to play soccer -- and score a goal -- he is fit enough to climb on to the sumo ring a hundred times!"

Asashoryu has tested the patience of sumo officials to breaking point with his bizarre behaviour since reaching sumo's elite rank of yokozuna, or grand champion, in 2003.

He infamously picked a punch-up in a bathtub with a rival wrestler, sparking mayhem as soapsuds flew and other grapplers were forced to step in to restore order.

JSA chairman Kitanoumi confirmed that he would be asking Asashoryu for an explanation of his latest breach of protocol.

"He handed in a doctor's note saying he's injured and the goes and plays soccer," Kitanoumi told reporters. "It's strange. We will need to check the facts."

Asashoryu, whose real name is Dolgorsuren Dagvadorj, was shown on Japanese television playing alongside Nakata despite telling the JSA he had a stress fracture in his lower back.