The evacuation of Britons from disaster-struck Japan will begin tomorrow evening, the Foreign Office (FCO) said tonight 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.
Meanwhile the Government's search and rescue team deployed to the country following the disaster called off its mission today and made plans to leave after finding no survivors in the wreckage.
Mr Jaczko's assessment of the crisis was notably bleaker than that made by the Japanese.
And while the US ambassador warned American citizens within 50 miles (80km) of the Fukushima Dai-ichi plant to leave the area or at least remain indoors, the Japanese government said it had no plans to expand its mandatory 12-mile (20km) exclusion zone around the plant.
Anyone within 20 miles (30km) of the site was urged to stay inside.
As foreign governments advised their citizens to leave the stricken region, Tokyo's Narita International Airport heaved with would-be passengers waiting anxiously for flights.
Akiko Takano, who works in information at the airport, said many had slept on floors and benches in the crowded terminals last night.
Some airport restaurants had to close early as food deliveries had not been made, due to problems on the roads, she said.
Ms Takano said: "Some flights are already full so people can't get on them but airlines are putting on extra flights. There are long queues at check-in but there's no confusion."
British Airways and Virgin Atlantic said their flights out of Tokyo were very busy but Virgin said planes were not completely full.
The first alternative option arranged by the British government is a Cathay Pacific flight leaving tomorrow evening from Tokyo to Hong Kong, on which up to 200 seats have been block-booked for British nationals.
Some 43 British and other eligible nationals had expressed an interest in this option by this afternoon.
Further flight options will be provided over the weekend as necessary including a charter on Saturday, the FCO said.
Chartered planes are also to be laid on for American citizens wishing to leave, the US State Department said.
Following the 9.0-magnitude earthquake and tsunami, more than 5,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000.
The troubles at the nuclear complex were set in motion by the natural disaster, which knocked out power and destroyed back-up generators needed for the reactors' cooling systems.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on the nuclear complex's damaged Unit 3 at 9.48am, defence ministry spokeswoman Kazumi Toyama said.
At least four loads were dumped on the reactor in the first 10 minutes alone, although television footage showed much of it appearing to disperse in the wind.
Defence minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.
Along with the helicopter water drops, military vehicles designed to extinguish fires at plane crashes were being used to spray the crippled Unit 3, military spokesman Mitsuru Yamazaki said.
But special police units trying to use water cannons failed in their attempt to cool the unit when the water failed to reach its target from safe distances, a spokesman for the Nuclear And Industrial Safety Agency said.
Officials at Tokyo Electric Power Co, which owns the plant, said they believed they were making headway in staving off a catastrophe both with the spraying and with efforts to complete an emergency power line to restart the plant's own cooling systems.