An inquiry into the Fukushima nuclear disaster has found that the crisis was preventable.

According to a panel of experts, the accident resulted from "collusion" among the government, regulators and the plant operator.

However, they also stated that the large earthquake and ensuing tsunami could not be ruled out as a cause of the accident.

Those involved in the investigation said regulators had been reluctant to adopt global safety standards that could have helped prevent the disaster.

The failure to do so caused reactors to melt down and spew out radiation.

The panel also highlighted problems in the way plant operator Tokyo Electric Power Co and the then Prime Minister Naoto Kan handled the crisis - which caused it to develop from a natural disaster into a man-made crisis.

Fukushima was the worst nuclear accident the world had seen for 25 years, around 150,000 people were forced to leave their homes, many of whom will never return.