There are "severe concerns" about Britons still missing in Japan four days after the devastating earthquake and tsunami, Prime Minister David Cameron has said.
There have so far been no confirmed reports of British fatalities in Friday's disaster, Mr Cameron told the House of Commons.
But around 17,000 UK nationals are known to have been in Japan at the time the catastrophic quake struck, and fears remain that some of them may be among the tens of thousands believed killed.
The Foreign Office's emergency helpline has been contacted by around 4,700 worried relatives and friends seeking news of loved ones. Foreign Secretary William Hague has urged any UK nationals who are in Japan or have recently left to make contact and confirm that they are safe.
Search and rescue teams struggling to save the lives of people still trapped in the debris have been joined by a 63-strong team from the UK.
Mr Cameron said Britain stood ready to send any further assistance that is needed - including nuclear experts to help tackle emergencies at plants including Fukushima, where cooling system failures at three reactors have led to hydrogen explosions.
In a statement to MPs, the Prime Minister said: "The devastation we are witnessing in Japan is of course of truly colossal proportions. We do not yet know the full and dreadful death toll, nor can anyone truly understand the impact these events will have, but Japan and the Japanese people are a resilient and resourceful nation. Britain and the British people are your friends and we have no doubt you will recover."
Fears of a major slowdown in the world's third-largest economy sparked a huge slump in Japanese shares, with Tokyo's Nikkei 225 index closing more than 6% lower and some of the world's biggest firms, such as Toshiba, Toyota and Honda, sustaining heavy share price losses. Shares in UK insurers also slumped amid estimates that the claims bill arising from the quake could reach as high as £22 billion.
After calls for a re-think of Britain's nuclear plans from environmental groups, Mr Cameron told MPs that any lessons from the incident should be learnt, but pointed out that the UK has no reactors of the type seen at Fukushima and no plans to build them.
Meanwhile, the US Geological Survey upgraded the magnitude of the earthquake to 9.0 from 8.9. The change means the quake was about 1.5 times stronger than initially thought and makes it the fourth largest in the world since 1900 behind the 2004 magnitude-9.1 Sumatra quake.