UN atomic safety experts said Japan underestimated the threat from a killer wave to its crippled Fukushima power plant.
It urged sweeping changes to prevent a repeat of the crisis that triggered the world's worst nuclear disaster since Chernobyl.
In a report presented to Prime Minister Naoto Kan today, an 18-member team from the International Atomic Energy Agency (IAEA) called for a rethink of the way nuclear facilities are built, run and regulated.
Officials have been criticised for failing to plan for a tsunami that would overrun the 5.7-metre wall at the plant in the northeast of the country.
This is despite forecasts from the government and Tokyo Electric Power Co's scientists that such a risk was looming.
The wave that crashed into the plant after the 9.0-magnitude earthquake of March 11 has been estimated at around 14 metres, nearly two-and-a-half times the height of the wall.
The IAEA report represented the first outside review of the crisis at Fukushima, which has still not been brought under control. It suggested that power companies in quake-prone countries may face higher hurdles in coming years to meet new international standards.
The report could also mark a new phase in a growing debate within Japan about how - and whether - the country's 49 remaining nuclear plants can return to operation.
"What we are trying to do is reflect on this and push for the highest safety standards worldwide," said Michael Weightman, Britain's top nuclear safety regulator and the leader of the IAEA inspection team.
The three-page IAEA report urged Japan to overhaul its system of nuclear regulation in order to make officials overseeing safety independent of the ministry that promotes nuclear power.
In 2007, the IAEA was ignored when it called on Japan to create a more powerful and independent nuclear regulator and clarify the responsibilities of the four government agencies with some responsibility for plant safety.
Goshi Hosono, a Kan aide who received the IAEA report, said a set of post-disaster reforms due to be announced as soon as next week would take up the question of how to make its nuclear regulatory agency independent.
"I think that the way nuclear plants are regulated will be taken up as one of the problem areas," Hosono told reporters.
Workers and officials have described a scene of near chaos when the quake and tsunami hit Fukushima on March 11. Key safety systems, including gauges and vents, were disabled and it was not clear in the crucial first hours who was responsible for decision making, according to accounts.
The IAEA said workers at Fukushima had been "dedicated" and "determined" but urged Japan to build "hardened" command centres to allow for a better response in future emergencies.
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