A new investigative report shows that Japan's leadership ordered workers to stay at the Fukushima nuclear plant last March as fears mounted that millions of people would have to evacuate Tokyo, 240km away.

Prime Minister Naoto Kan and his staff began referring to a worst case scenario about three days after the March 11 earthquake, tsunami and nuclear disaster, says the report by a panel set up by a private think-tank. The Rebuild Japan Initiative Foundation says that was when fears mounted that thousands of spent fuel rods stored at a damaged reactor would melt and spew radiation.

Yukio Edano, the government spokesman, feared a "devil's chain reaction" in which the Fukushima plant, the Fukushima Daini facility and Tokai nuclear plant, spiralled out of control, putting Tokyo at risk.

- agencies