Japan underestimated the risk of a tsunami hitting a nuclear power plant, the UN nuclear energy agency has said.
However, the response to the nuclear crisis that followed the 11 March quake and tsunami was "exemplary", it said.
The Fukushima Daiichi nuclear plant, which was badly damaged by the tsunami, is still leaking radiation.
Japan's Prime Minister Naoto Kan is facing a no-confidence vote submitted by three opposition parties over his handling of the crisis.
They say he lacks the ability to lead rebuilding efforts and to end the crisis at the Fukushima plant, public broadcaster NHK reported.
Some politicians from Mr Kan's governing Democratic Party of Japan (DPJ), including former Prime Minister Yukio Hatoyama, are backing the motion.
If it is passed in a vote expected on Thursday, Mr Kan would be forced to resign or call a snap election.
The IAEA inspectors spent a week in Japan compiling their report on the Fukushima nuclear crisis, and have handed a summary to the Japanese government.
The full report, intended to improve nuclear safety around the world, will be submitted at an inter-governmental meeting in Vienna later this month.
The UN team was led by Britain's top nuclear safety official Mike Weightman and included experts from France, Russia, China and the United States.
The disaster in numbers
The inspectors pointed out a key failure, already admitted by Japan, to plan for the risk of waves crashing over the sea wall and knocking out the plant's back-up generators.
Even though a major faultline lies just offshore, the sea wall at Fukushima was less than 6m (20ft) high. The height of the tsunami wave was about 14m.
"The tsunami hazard for several sites was underestimated," the UN team's three-page preliminary report said.
"Nuclear plant designers and operators should appropriately evaluate and provide protection against the risks of all natural hazards."
Amid concerns about safety, just 17 of the country's 54 reactors are operating.
The IAEA said continued monitoring of the health and safety of the nuclear workers and the general public was necessary.
The report also emphasised the importance of independent regulators in the nuclear industry.
In Japan, the Nuclear Safety Agency is part of the industry ministry, which promotes nuclear power.
"The planned road map for recovery of the stricken reactors is important and acknowledged. It will need modification as new circumstances are uncovered and may be assisted by international co-operation," the IAEA report said.
Goshi Hosono, an aide to Prime Minister Kan, accepted the report and said the government would need to review its nuclear regulatory framework.
The government invited the IAEA to Japan to demonstrate that lessons are being learned from what happened at Fukushima, says the BBC's Roland Buerk in Tokyo.
The powerful earthquake and the tsunami it generated are now known to have killed more than 15,280 people, while nearly 8,500 remain unaccounted for.