The US nuclear energy watchdog ordered a Californian atomic plant Tuesday to remain shut down until the cause of a mystery glitch is found, raising questions about high-season power supplies.

The San Onofre nuclear plant north of San Diego -- one of only two in the populous western state -- has been offline since January, when one of its units suffered a leak in a steam generator tube.

The incident triggered checks which found unexpected decay on tubing which carries radioactive water -- and on Tuesday the Nuclear Regulatory Commission (NRC) told its operators to fix the problem fully before re-starting the plant.

Power company Southern California Edison must "ensure that the cause of the tube wear in both steam generators is understood and appropriately addressed in order to ensure safe operation," said NRC official Elmo E. Collins.

"Until we are satisfied that has been done, the plant will not be permitted to re-start," he added, in an NRC statement.

The power firm pledged to comply fully with the regulators' demands. "We welcome the NRC's letter, which is a formal step in the process of restarting Units 2 and 3," said Edison chief Ron Litzinger.

"Our number one priority is, and always has been, the health and safety of the public and our employees. The utility will only bring the units on line when we and the NRC are satisfied that it is safe to do so."

It is unclear how long it will take to resolve the problem, but questions have been raised about the impact if it continues into the summer, when power demand is highest as Californians turn on their air-conditioners.

In a major heat wave or transmission line outage in the peak season, southern California could face energy shortages without the 2,200 megawatts of power generated by San Onofre, the Los Angeles Times newspaper reported.

California has one other nuclear power plant, the Diabolo Canyon facility between Los Angeles and San Francisco, operated by Pacific Gas and Electric.

Edison said safety must come first at San Onofre, pledging it "will proceed deliberately and conservatively to implement these steps (ordered by the NRC), always bearing in mind that safety is our first priority."