The Japanese government withheld information about the full danger of last year’s Fukushima nuclear plant disaster from its own people and from its key ally, the United States, an independent investigation has found.

A map locating the epicentre of the earthquake that caused the Japan tsunami on March 11, 2011 and triggered meltdowns at the Fukushima nuclear plant.

A worst-case scenario sketched out by the government foresaw the end of Tokyo in a chain of nuclear explosions that would mean evacuating the city.

Plans were drawn up for the mass withdrawal from the capital as at least one senior minister fretted that meltdowns at Fukushima might spark crises at reactors all along the coast and engulf the city of 13 million people.

The revelations came in a 400-page report published by a panel of experts who were given free rein to probe the events surrounding the world’s worst nuclear crisis in a generation.

“I had this demonic scenario in my head that nuclear reactors could break down one after another,” then chief Cabinet secretary Yukio Edano told the panel.

“If that happens Tokyo will be finished,” he said, according to the report.

The panel said as the situation on Japan’s tsunami-wrecked coast worsened, Fukushima operator Tokyo Electric Power (Tepco) had wanted to abandon the plant and evacuate its workers.

But the utility, which refused to cooperate with the study, was ordered to keep men on site by then Prime Minister Naoto Kan.

Experts concluded that if the premier had not stuck to his guns, Fukushima would have spiralled further out of control, with catastrophic consequences.

“When the Prime Minister’s office was aware of the risk the country may not survive (the crisis)...Tepco’s president (Masataka) Shimizu....frantically called to tell the premier he wanted his staff to leave the crippled nuclear reactor,” panel head Koichi Kitazawa told a news conference.

Mr Kitazawa said Mr Kan threatened to break up the powerful utility if the company insisted on pulling its men out.

He said Mr Kan’s refusal to bow to Tepco’s demand had averted a worse crisis.

Mr Kan told Mr Shimizu: “It’s impossible. If you withdraw staff, Tepco will be demolished,” according to Mr Kitazawa.

“Consequently, it’s Mr Kan’s biggest contribution that the Fukushima 50 remained at the site,” added Mr Kitazawa, referring to dozens of operatives who worked to contain the accident and were feted as heroes.

Respected academics, engineers and journalists were drafted in by the Rebuild Japan Initiative Foundation after public demands for an independent probe into the meltdowns at Fukushima in the aftermath of the monster tsunami of March 2011.

The six-member panel was given access to data and documents used in the days and weeks after disaster struck.

The panel said Mr Kan had instructed experts to draft a plan to evacuate a huge swathe of the country, based on the worst case scenario.

Planners worked on the assumption that if the nuclear crisis were to worsen “it is possible that a compulsory evacuation zone will spread to 170 kilometres ... and a voluntary evacuation zone will spread to 250 kilometres and beyond”.