The Foreign Office has advised Britons to consider leaving Tokyo and north-east Japan following Friday's quake, tsunami and subsequent radiation fears.
British officials report there is still "no real human health issue that people should be concerned about".
The disaster sparked a nuclear emergency at a stricken power plant, 220km (140 miles) north of Tokyo.
The Foreign Office says its advice is based on the situation at Fukushima plant, and food and fuel shortages.
Foreign Office Minister Jeremy Browne said the advice was "not an order" but that given the situation "British nationals should consider leaving Tokyo and northern Japan".
He said they should be able to leave on a plane, or at least travel to other parts of the country by train or bus, but "if the capacity needs to be increased by the British government, we will do that".
"We are informed by the science, and the science says that outside the 30km exclusion zone there should not be a threat to human health," he added.
"If we thought there was a threat to human health of a severe level in Tokyo, we would have much stronger advice at this stage, saying that people should leave with immediate effect."
The Foreign Office statement said: "Due to the evolving situation at the Fukushima nuclear facility and potential disruptions to the supply of goods, transport, communications, power and other infrastructure, British nationals currently in Tokyo and to the north of Tokyo should consider leaving the area."
The travel advice has changed from "advising against all non-essential travel to Tokyo and north-eastern Japan".
A meeting of the UK government emergency response committee, Cobra was held on Wednesday evening to discuss the situation in Japan.
Elsewhere, France is urging its nationals in Tokyo to leave the country or move south as two Air France planes are sent there to begin evacuating people.
The US is warning its citizens living within 50 miles (80 km) of the plant to evacuate or seek shelter, and is barring US troops from the area.
Australia is advising its people to consider leaving Tokyo and the most damaged prefectures, while Turkey is warning against travel to Japan.
The 9.0-magnitude quake and tsunami struck on Friday, killing thousands and leaving more than 500,000 people homeless amid freezing temperatures and aftershocks.
UK nationals are being asked to confirm their safety by contacting the Foreign Office (FCO) on +44 (0)20 7008 0000. That is also the helpline number for people concerned about friends and relatives in Japan, and the FCO said 5,480 people had called it as of Tuesday night.
A bus has been organised to take Britons from Sendai to Tokyo on Thursday. It will leave from the consular response centre at the ANA Holiday Inn at midday local time (0300 GMT).
Passengers wanting to register should call the British embassy in Tokyo on (+81) 80 3250 2924 or (+81) 35 211 1356 if calling from the UK, or 080 3250 2924 or 035 211 1356 if calling within Japan.
Most of the 17,000 Britons living in Japan are understood to be in Tokyo or Osaka.
David Warren, British ambassador to Japan, told BBC Radio 5 live he was not going to speculate about exact numbers of British casualties because the figures were constantly changing.
He said the embassy had a list of Britons believed to have been in the quake-hit areas and embassy staff had been able to eliminate an "enormous" number of people by phone or e-mail.
"There are still people we have been unable to locate," he added.
"Although we don't have any confirmation of British casualties, the size of this disaster must mean there is a probability there will have been some."
Earlier, it was claimed UK rescue workers had to leave Japan because they did not have the necessary paperwork from the British embassy in Tokyo, a claim denied by the Foreign Office.
The International Rescue Corps said they were not given permission to work in the country because it would have made the embassy legally responsible for them.