A government-appointed inquiry into Japan's Fukushima nuclear crisis raised doubts on Monday about whether other atomic plants were prepared for massive disasters despite new safety rules, and delivered a damning assessment of the regulators and the station's operator.

The report, the second this month about the disaster, could be seized upon by Japan's increasingly vociferous anti-nuclear movement after the restart of two reactors, and as the government readies a new energy policy due out next month.

The panel suggested post-Fukushima safety steps taken at other nuclear plants may not be enough to cope with a big, complex catastrophe caused by both human error and natural causes in a "disaster-prone nation" like Japan, which suffers from earthquakes, tsunami and volcanoes.

"We understand that immediate safety measures are being further detailed and will materialize in the future. But we strongly urge the people concerned to make continued efforts to take really effective steps," said the panel, chaired by University of Tokyo engineering professor Yotaro Hatamura.

Tokyo Electric Power Co (Tepco), the operator of the Fukushima Daiichi plant, and regulators failed to plan for a massive natural disaster, the panel said, blaming them for being lulled by the same "safety myth" blasted by a parliament-appointed team of experts earlier this month.