Japan crisis: Britons offered flights to Hong Kong
The UK government is chartering planes to fly from Tokyo to Hong Kong to help Britons who want to leave Japan.
The Foreign Office said there would be no charge for Britons "directly affected" by the tsunami, but a charge of £600 would otherwise apply.
People are being advised to still use commercial flights if possible.
The prime minister's spokesman said Britons were advised to stay outside an 80km (50 mile) exclusion zone around the Fukushima nuclear plant.
The UK's Scientific Advisory Group for Emergencies (Sage) is continuing to meet and monitor the situation in the troubled reactor.
The spokesman said Sage had examined worst-case scenarios and risks to human health and concluded the risk could be managed if people stayed indoors.
Foreign Office minister Jeremy Browne said the official advice to Britons in Tokyo was that they should "consider" leaving.
He told the BBC: "If we thought there was a really serious risk to human health in Tokyo, we would be saying so and we would be evacuating. We are not in that stage.
"But if we thought there was no risk whatsoever to any level at all, we'd also be saying that. We think that there is, potentially, in the very worse case scenarios, a small risk of a small amount of exposure."
Foreign Secretary William Hague told the House of Commons there were "severe concerns" about some British nationals who officials have been unable to locate.
"Our consular teams in London and Japan have been working round the clock," he said. "We're following up all the leads from the helpline we have set up."
Mr Hague said more than 50 extra staff have been sent to the affected regions, and they are visiting reception centres and hospitals.
About 17,000 British nationals were believed to be in Japan last Friday but there have been no reports yet of fatalities.
Japan's east coast has been devastated by a tsunami which was triggered by an earthquake last Friday.
Those areas not wiped out are suffering power cuts and water shortages.
Marc Kemp, who works as an English teacher in Fukushima, said he had to bear in mind fuel shortages while planning to possibly leave the area.
He told the BBC: "It's really forced us to re-evaluate how we would do these things, and last night we all sat down and worked out the logistics.
"We do have a plan in mind, we know where we would go, where we know we have enough fuel, food and water to get where we would want to go."
A Foreign Office statement said it continued to advise against all non-essential travel to Tokyo and north-eastern Japan and British nationals currently in Tokyo and to the north of Tokyo should consider leaving the area.
Coach laid on
A Foreign Office spokesman added: "The UK government is chartering flights from Tokyo to Hong Kong to supplement commercially available options for those wishing to leave Japan.
"Commercial flights continue to operate to and from Japan. British nationals with commercial flight bookings should continue to use these flights and you should continue to make reservations and other arrangements with your airline as usual.
"If you wish to leave and cannot secure tickets for a commercial flight, you can register your interest in using our charter option to leave Japan by calling 44 20 7008 6900."
The British embassy has also organised a coach to take Britons from Sendai to Tokyo.
British rescuers are still working to find survivors and bodies in the Tohoku region.
One of the rescuers, Alan Downes, said: "We've seen total devastation in the areas affected. It's so significant, I struggle to comprehend how any community can move on from it.
"But people here are getting on and dealing with what's put in front of them."