The Japanese government feared that millions of Tokyoites might have to be evacuated during the worst of last year's nuclear crisis, but kept the scenario secret to avoid panic in some of the world's most crowded urban areas, according to an internal report.

The 15-page report, by the Japanese Atomic Energy Commission, was delivered to the then-Prime Minister, Naoto Kan, two weeks after the 11 March earthquake and tsunami triggered the crisis at the Fukushima Daiichi nuclear plant.

It warned that if the situation spiralled out of control, compulsory or voluntary evacuation orders would have to be issued to residents living within 250 kilometres (155 miles) of the damaged facility, a radius that would have included the Tokyo metropolitan area that is home to around 30 million people.

The directive would also have covered several large cities to the north and west of the plant, including Sendai. Some of the areas would be contaminated for "several decades", the report warned.

Last May, Fukushima's operator, Tokyo Electric Power Company (Tepco), admitted that uranium fuel inside three of the plant's reactors had melted down in the early days after disaster struck. A series of hydrogen explosions had showered thousands of square miles of land and sea with radioactive substances but officials from the government and Tepco repeatedly denied the meltdown scenario.

More than 80,000 people were subsequently told to leave the most heavily irradiated areas around the nuclear plant and have yet to return. Tens of thousands more have since left Fukushima prefecture voluntarily.

Mr Kan and his government insisted throughout March and April that the nuclear crisis was being contained and ignored calls to widen the evacuation area. But after he left office, the Prime Minister admitted in a newspaper interview that he feared the Fukushima disaster would leave the capital uninhabitable, and that evacuating it would have been "impossible". He said the "spine-chilling thought" of a deserted capital convinced him to scrap nuclear power.

The latest revelations will revive criticism that the authorities have been less than forthcoming since the crisis erupted, and add to suspicions that they are still downplaying the impact of radiation. Government officials recently admitted that data on where the radiation went was withheld from the Japanese public for 10 days, even though it was shared with the US military in Japan.

The report will also add to concerns that Japan is unprepared for a similar disaster. Last week, researchers at the University of Tokyo warned that there is a 75 per cent probability that the capital will be hit by a major earthquake in the next four years.