Britain has offered assistance to Japan after it was hit by a massive earthquake which triggered a tsunami.
Foreign Secretary William Hague said the UK could help with humanitarian assistance or search and rescue teams. There were no known British casualties, he said.
The Foreign Office has advised against all non-essential travel to Tokyo and the north-east of the country.
The Queen has sent a message of sympathy to the people of Japan.
Buckingham Palace said she had expressed her sadness at the "tragic loss of life" to Emperor Akihito. Prince Charles also wrote to the emperor to say the news had come as a great shock.
Prime Minister David Cameron also offered the Japanese people his condolences and said "we stand ready to help in any way that we can".
The earthquake hit the north-east of Japan at 1446 local time (0546 GMT) on Friday, triggering a tsunami. Hundreds are known to have died so far but the death toll is expected to rise significantly.
It has emerged that BBC Philharmonic Orchestra members are safe after they were caught up in the 8.9-magnitude earthquake as they travelled from Tokyo to Yokohama.
Orchestra manager Fiona McIntosh said it had been a "terrifying experience".
Another Briton, Mike Hall, who lives in Hokkaido and is originally from Newport, south Wales, told the BBC: "Initially, I didn't pay any attention as we've had earthquakes before - but this time, it went on for longer and there were several big aftershocks."
Matthew Holmes, a 27-year-old from Nottingham, was at work in Shimokitazawa, west-central Tokyo, when the earthquake hit.
He described the sensation as "like many shocks, joined up by a feeling of being on a wave".
Search and rescue teams from the UK - International Rescue Corps and Rapid UK - say they are on standby having both formally offered their services.
International Development Secretary Andrew Mitchell said they were on four hours' notice to fly to the disaster zone.
"It is clear Japan has some of the most sophisticated search and rescue people in the world, but if we are asked for any technical or additional support, then of course we will give it," he told BBC Radio 4's World at One.
The charity Shelterbox - which assembles boxes containing tents, bedding and other essentials - has sent two officials to Japan to assess the need for its help.
Search and rescue teams from 45 countries are said to be ready to help. The first to arrive in Japan will be from the US, Australia, New Zealand and South Korea, the UN says.
The Foreign Office has updated its travel advice for Britons in Japan.
A spokesman said the FO "advises against all non-essential travel to Tokyo and the north-east of Japan, whilst we assess the damage caused".
More than 17,000 British nationals live in Japan, most of whom are based in Tokyo and the western city of Osaka. A further 300,000 UK nationals visit the country each year.
Mr Hague earlier chaired a meeting of the UK's Cobra emergency response committee.
He spoke to his Japanese counterpart on Friday afternoon to express his sympathy for the loss of life and to offer help, the Foreign Office said. He also requested any information about British nationals caught up in the disaster.
Meanwhile, embassy and consular staff across Japan have been in touch with local authorities and have been trying to make contact with British nationals.
Another consular team of eight are due to fly into Japan from Hong Kong on Saturday to help embassy staff. Other consular teams are on standby in London and the US, the Foreign Office added.
British ambassador David Warren said efforts to contact British nationals living outside the main cities had been hampered by heavily restricted phone lines.
However, lines were beginning to open up again, he said, and they were not yet aware of any British nationals caught up in the devastation.
The Foreign Office says British nationals and friends and relatives of those in Japan should contact it in the UK on +(44) 207 008 0000.
Meanwhile, British Airways cancelled its daily Heathrow service to Tokyo's Narita airport on Friday as well as services out of Tokyo on Saturday.
Virgin Atlantic, which operates daily services to Tokyo from Heathrow, has cancelled flights on Friday and over the weekend.