Radioactive fallout from Japan's crippled nuclear plant has reached the US, with experts saying there is a small chance it could soon reach Britain.
Readings of the radiation in southern California are said to be far below levels which could pose a health hazard, however.
And the UK's Health Protection Agency stressed there was no health risk to British residents from the release of radioactive material from the stricken Fukushima Dai-ichi plant.
Meanwhile, the evacuation of Britons from the disaster-hit country gathered pace as the situation at the dangerously overheated nuclear facility continued to deteriorate.
The emergency was sparked by last Friday's 9.0-magnitude earthquake and tsunami in north-east Japan, which has left more than 6,500 dead and over 10,300 missing.
A US diplomat with access to radiation tracking by the UN's Comprehensive Test Ban Treaty Organisation (CTBTO) cited initial readings from a California-based measuring station. These were "about a billion times beneath levels that would be health threatening", he said.
The Nuclear Energy Agency, part of the international Organisation for Economic Co-operation and Development, said radiation from the plant was likely to reach everywhere in the northern hemisphere but in extremely small quantities.
Meteogroup, the weather division of the Press Association, said it was possible but unlikely that the radiation would cross Europe in the next week or two. Forecaster Stephen Davenport said: "It of course remains a possibility, however, as weeks progress and wind directions change in the weeks to come after."
As efforts to bring the situation under control continued, military fire trucks sprayed water on overheating reactor units at the plant for a second day. Buses and planes ferried people to safety, with 24 British nationals leaving tsunami-flattened Sendai at midday local time on two coaches heading for Tokyo.
The Foreign Office (FCO) block-booked seats for Britons wanting to fly home on commercial flights, the first of which was a Cathay Pacific flight to Hong Kong. The FCO said two other flights would be made available on Saturday - a Hong Kong Airlines flight and an Orient Thai Airlines plane, both destined for Hong Kong.