A third explosion last night at Japan's quake-stricken Fukushima nuclear plant has led to radioactivity being released directly into the atmosphere marking a dramatic escalation in the crisis and threatening the nation's 12 million populated capital, Tokyo, 155 miles away, with low-level radiation.

Japanese Prime Minister, Naoto Kan, said the nuclear plant 'remains in an alarming state' with radiation around the facility having 'risen considerably' after the further blast.

'I know that people are very worried, but I would ask you to act calmly,' urged Kan.

Meanwhile, the French embassy has warned its citizens to leave the capital and many multinational companies are drawing up urgent plans to temporarily relocate.