The plant operator had been warned the pipe was a safety risk

Kansai Electric Power admitted it was told last year that a cooling pipe which burst was a safety threat.

The pipe was not checked again because it was not expected to corrode so quickly, and it had not been thoroughly checked since 1976, the company said.

Officials insist there was no radiation leak following the accident.

Four people were killed and seven injured by escaping steam and boiling water after the pipe burst in the plant in Mihama, Fukui prefecture. At least one of the injured is in a critical condition, with 80% burns.

It was the deadliest accident that a Japanese nuclear power plant has suffered, and has again rocked confidence in the country's accident-prone nuclear industry.

Police investigators were accompanied by regional and national authorities as they arrived to inspect the plant on Tuesday, said police spokesman Fuminaga Miyamoto.

"Police are investigating the company on suspicion of corporate negligence resulting in death," he said.

The company, also known as Kepco, has already admitted that the cooling pipe had dangerously corroded to just 1.4mm from its original 10mm thickness. It said it has not properly inspected the pipe since it was fitted in 1976.

"We conducted visual inspections, but never made ultrasonic tests, which can measure the thickness of a steel pipe," said spokesman Haruo Nakano.

After the accident, Kepco found a hole in the pipe, through which steam from 150 degrees Celsius- (300 Fahrenheit-) water had spewed.

Safety check due

Japan's Kyodo news agency cited investigation sources as saying that police believe Kepco may have neglected safety standards by allowing workers to prepare for an annual inspection while the plant was still running.

The inspection was due to commence on Friday. Nuclear reactors are supposed to be shut down for inspections, Kyodo said.

Kepco deputy plant manager Akira Kokado said the company had been told by private contractors in April 2003 that the cooling pipes needed a thorough safety check.

The examination had been scheduled for 14 August - this coming Saturday.

"We thought we could delay the checks until this month," Mr Kokado said.

"We had never expected such rapid corrosion."

The BBC's Tokyo correspondent, Jonathan Head, says the government is now asking the operators of 22 other nuclear power plants similar to the one in Mihama to check their past inspection records.

Kepco spokesman Kenji Yamashita told BBC News Online that his company's 11 nuclear plants would all be checked immediately, and would be closed down if necessary.

Japan's Trade Minister Shoichi Nakagawa, who is responsible for nuclear policy, apologised on Tuesday for the accident.

"We must not undermine trust in nuclear energy policy. We would like to investigate the cause and make sure it does not happen again," he said.