Police in Japan have been left red-faced by an apparent murder that turned out to be an unusual case of mistaken identity.

It began in the morning with a frantic call from a couple who had spotted a "corpse" while out walking their dog in a mountain forest in Izu, central Japan, the ZakZak news website reported today.

Fifteen officers were dispatched to the scene, where they discovered a human form wrapped in plastic and tightly bound around the neck, midriff and ankles, with hair protruding from one end.

The body was left untouched and taken away for examination, and the crime scene duly secured by a police cordon.

Back at the local police headquarters, officials notified reporters who had turned up early the same morning to cover an annual earthquake drill. They began preparing to write up the launch of a major murder investigation.

Dozens of extra officers were dispatched to interview potential witnesses, while the evening edition of the local newspaper carried a report of the gruesome find, complete with a photograph of the body's resting place.

By mid-afternoon, the body was in the hands of police pathologists. But when they sliced open the wrapping, they were confronted not by a decomposing corpse, but by a life-sized sex doll.

A police spokesman apologised for the commotion but defended his officers, saying they had simply been following protocol by leaving the concealed "body" untouched until it was in the hands of pathologists.

Though no crime had been committed, the spokesman could not resist admonishing the doll's mystery owner. The doll, he told bemused reporters, showed signs of repeated use.

"Our guess is that the owner didn't want to take a risk by throwing it away with the rest of his rubbish," he said. "It was an incredibly irresponsible thing to do."