TOKYO (Reuters) - Japan's prime minister ordered workers to remain at the tsunami-crippled Fukushima nuclear plant last March as fears mounted of a "devil's chain reaction" that would force tens of millions of people to flee Tokyo, a new investigative report shows.

Then-premier Naoto Kan and his staff began referring to a worst case scenario that could threaten Japan's existence as a nation around three days after the March 11 disaster, according to the report by a panel set up by a private think-tank.

Yukio Edano, then Japan's top government spokesman, told the panel that at the height of tension he feared a "devil's chain reaction" in which the Fukushima Daiichi plant and the nearby Fukushima Daini facility, as well as the Tokai nuclear plant, spiralled out of control, putting the capital at risk.

Kan, who stepped down last September, came under fire for his handling of the crisis, including flying over the plant by helicopter the morning after the disasters hit -- a move some critics said contributed to a delay in the operator's response.

In an interview with Reuters this month, the 65-year-old Kan said he was haunted by the spectre of a crisis spiralling out of control and forcing the evacuation of the Tokyo greater metropolitan area, 240 km (150 miles) away and home to some 35 million people.

Confused media reports at the time of the accident said Fukushima plant operator Tokyo Electric Power Co, known as Tepco, had threatened to withdraw workers from the plant, but that Kan ordered them to keep staff on-site.

"Now Tepco is saying there was no request for a complete pullout, that it only asked for a partial withdrawal. The truth may never come out, but as a result, 50 Tepco staff stayed behind and ... the worst case scenario was averted," panel chief Koichi Kitazawa told Reuters before the report's release.

How many of those who stayed were volunteers is a mystery.

"An order was likely given for full-time employees to stay behind. We may eventually find out who volunteered to stay, but the impression from our investigation is that they are under strict orders to remain silent."

"STRONG SENSE OF CRISIS"

The six-member panel is one of several probing the disaster caused by the March earthquake and tsunami that knocked out cooling systems at the Fukushima Daiichi plant and triggered meltdowns of nuclear fuel in the worst radiation crisis since Chernobyl.

Since September, it has interviewed more than 300 people, including Kan, then-trade minister Banri Kaieda and Edano.

Kan's administration, Tepco and nuclear regulators have all faced criticism, both for a confused response and for failing to come clean on the extent of the crisis in the early days, undermining public trust in Japan's leaders and bureaucracy.

Edano on Tuesday acknowledged he had feared the worst around March 14-15. "I was working with a strong sense of crisis that under various circumstances, such a thing may be possible," he told a news conference.

But he defended his silence as government spokesman.

"I shared all information. Back then, I was not in a position where I, as someone who is not an expert, could irresponsibly speak about my own personal impressions and my sense of crisis," he told a news conference.

"I conveyed assessments and decisions of the government, government agencies and experts," he added.

The panel report said some of Kan's seemingly inexplicable behaviour stemmed from his belief that Tepco was going to abandon the plant and the accident would spiral out of control.

"We can begin to understand some of Kan's actions, which at first appeared inexplicable, when considering the presence of the worst case scenario," Kitazawa said.

An irate Kan blasted Tepco on March 15, yelling: "What the hell is going on" in an outburst overheard by a Kyodo news reporter and quickly reported around the globe. "I want you all to be determined," he was quoted as telling utility executives.

The utility ultimately left a corps of workers who were dubbed the "Fukushima Fifty" by media and won admiration at home and abroad as they risked their lives to contain the crisis.

Their names were not made public, a reticence some attributed to cultural norms emphasising the group and others to the antipathy to Tepco that emerged as a result of the accident.

After the quake and tsunami struck, three reactors at the Fukushima plant suffered meltdowns and radiation spewed widely through eastern Japan, forcing tens of thousands of residents to evacuate from near the plant and contaminating food and water.

Tepco managed to avert a worst case scenario by pumping water, much of it from the sea, into Daiichi's damaged reactors and spent fuel pools. The reactors were stabilised by December.

(Additional reporting by Yoko Kubota; Editing by Linda Sieg and Alex Richardson)