Their report said that the meltdowns had been the result of collusion between the government, regulators and privateer Tepco that allowed lax preparations and shoddy precautions.
It said the response “betrayed the nation’s right to be safe from nuclear accidents.”
The 10-member panel, set up in December, interviewed 1,167 people in 900 hours of hearings.
Its final report urged parliament to keep a close eye on regulators and crisis management reforms.
It also urged the government to set clear disclosure rules about its relationship with nuclear operators, construct a cross-monitoring system and get nuclear energy laws up to “global standards of safety, public health and welfare.”
The report could fuel complaints that Japan is trying to restart nuclear reactors without doing enough to avoid a repeat, especially since Kansai Electric Power restarted the Ohi No 3 reactor just hours before the report was issued, with enthusiastic backing from the government of Prime Minister Yoshihiko Noda.
Experts and activists have criticised the government, saying that it is putting business ahead of safety by letting reactors be restarted before properly studying the report.
Other groups have also issued lengthy studies detailing a serious lack of communication between the government and Tepco along with a failure by both to provide the public with important information on radiation leaks.
The commission’s report is likely to complicate government efforts to get more reactors going.
Over the past month, large demonstrations against restarts have been held each week outside the prime minister’s office, reflecting deep grass-roots opposition.
Before the crisis, Japan got one-third of its electricity from nuclear plants.
The government hopes to see the restart of more of Japan’s 50 reactors as soon as possible.