Japan has requested help from the UK in the aid effort after yesterday's magnitude 8.9 earthquake rocked the country, a Foreign Office spokesman said.
Hundreds of people were killed and many more are missing after the massive quake struck, sending a wall of water across the Pacific.
At around 8,000 times more powerful than the one which devastated Christchurch in New Zealand last month, the earthquake was one of the largest ever recorded.
The request for assistance from Britain comes as an explosion tore through a nuclear power plant in one of the worst-hit areas.
Footage on Japanese TV showed the walls of one building at the Fukushima Dai-ichi plant had crumbled, leaving only a skeletal metal frame block standing. Puffs of smoke were seen spewing out of the plant.
Pressure had been building up in the reactor and Japan's Nuclear and Industrial Safety Agency told reporters today that it was venting "radioactive vapours" to relieve that pressure. Officials said they were measuring radiation levels in the area.
Operators at the plant's Unit 1 detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
The rescue and aid teams in the UK were put on standby hours after the quake hit, and now that Japan has officially requested their assistance, they will start their journey.
Yesterday's quake hit at 2.46pm (5.46am GMT) local time, unleashing a tsunami that reached heights of more than 20ft and crashed into the eastern coast of Japan. The quake was followed by at least 12 powerful aftershocks,
It struck at a depth of six miles about 80 miles off the eastern coast, about 240 miles north east of Tokyo.
British Foreign Secretary William Hague said: "We are all appalled by the scenes of devastation, by the heavy loss of life, by the destruction we have all witnessed on our television screens.
"I think all over the world, people's hearts go out to the people of Japan.
"I spoke to the Japanese foreign minister today to convey our condolences, and also our offer of help."
Mr Hague said the UK was able to offer assistance in the form of search and rescue teams or victim identification expertise.
He said: "We are discussing that with the Japanese government.
"They have now asked for some help. We're nailing down the details for that and a further announcement of that will be made later today."
The Foreign Secretary said the Government was "concerned" about British citizens who may have been in the affected area, particularly in north-eastern Japan.
He said: "We have no confirmed British casualties as yet, but of course the picture will become clearer as recovery teams do their work, as communications are re-established in the affected area."
The Government has sent reinforcements to the British embassy in Tokyo, he said.Reuse content