The evacuation of Britons from disaster-struck Japan will begin on Friday evening, the Foreign Office (FCO) has said amid a warning the nuclear situation is deteriorating.
Increasingly frantic attempts have been made to bring the nuclear emergency under control with water cannons and helicopters dropping seawater to cool an overheated complex.
US Nuclear Regulatory Commission chairman Gregory Jaczko said anyone close to the Fukushima plant, which has been badly damaged by the earthquake and tsunami, would be at risk.
"We believe radiation levels are extremely high," he said.
As queues grew at check-in desks in Tokyo airport terminals, the FCO said it had arranged a number of options for British nationals unable to book seats on commercial flights.
These include charter flights to Hong Kong, as well as seats on commercial planes the Government has block-booked.
The offer of help came after Britons were advised to consider leaving Tokyo amid desperate attempts by the Japanese to prevent meltdown at the Fukushima Dai-ichi nuclear plant.
Foreign governments were also scrambling to fly their citizens out.
A French government plane has already left Tokyo for Seoul in South Korea with 241 people on board, with another flight planned for French citizens wishing to leave Japan.
And two Czech military planes landed in Prague after evacuating 106 people from the country.
