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JAPAN increased the nuclear threat level yesterday as experts predicted an airborne radioactive plume could reach Britain within two weeks.

Dangerous levels of radiation continued to spew into the atmosphere from the stricken Fukushima power station and the country’s nuclear safety agency raised the alert rating from four to five – on par with the 1979 Three Mile Island incident in the USA.

Only the devastating explosion at Chernobyl in 1986 has topped the scale at seven.

Particles from the Fukushima plant have already been traced on planes arriving in the US from the disaster-torn country.

And a plume of radiation reportedly hit California yesterday before being swept across America towards Europe.

But experts stressed the levels would not be high enough to pose any human health risks.

The boss of the company which owns the plant broke down in tears yesterday as the level five emergency was announced – signifying a high probability of “significant” public exposure or several deaths from radiation.

Tokyo Electric Power Company managing director Akio Komiri cried as he left a conference to brief journalists on the situation.

The Japanese government was facing growing international condemnation for its handling of the disaster and the nation’s chief cabinet secretary Yukio Edano admitted: “The unprecedented scale of the earthquake and tsunami were among many things that happened that had not been anticipated under our disaster management contingency plans.”

And Britain’s Energy Secretary Chris Huhne yesterday said he had asked chief nuclear inspector Mike Weightman for an assessment of our nuclear safety by mid-May, with a final report to be published within six months.

The Fukushima rating applies to reactors one, two and three hit by fire and explosions since the plant was smashed by a tsunami triggered by last week’s earthquake.

The UN’s atomic energy chief Yukiya Amano said the emergency was now a race against time and stressed: “This is not something that just Japan should deal with. People of the entire world should co-operate with Japan.”

Lars-Erik De Geer, of the Swedish Defence Research Institute, said particles would eventually be detected in Britain. But he added: “It is not high from any danger point of view so it is nothing for people to worry about.”

Workers at Fukushima yesterday continued their battle to prevent a meltdown in what is already the world’s worst nuclear disaster since Chernobyl in the old Soviet Union – where death estimates vary from 4,000 to 730,000.

After the partial meltdown at Three Mile Island in Pennsylvania, an investigation concluded there would either be no resulting cancer deaths or the number would be undetectable. But the accident marked a decline in the number of new reactors then being built.

And officials in Japan are now rapidly running out of options to halt their crisis.

The overheating reactors were sprayed with tons of water for a second day but burying them under concrete may be the only option.

Engineers succeeded in restoring power to one of the cooling systems but it was not certain it would work after such extensive damage.

A 12-mile evacuation zone has been declared around the plant and 140,000 people living within 18 miles have been told not to go out.

Spain has now joined Britain, the US and several other countries who are evacuating their concerned citizens from Japan.

And panic has spread overseas, with shops in parts of the US being stripped of iodine pills, which can protect against radiation, and Asian airports scanning passengers from Japan for possible contamination.

Yesterday Japan held a minute silence exactly a week after the 9.0-magnitude earthquake.Police say 6,911 people are known to have died and 10,316 are missing but some say the death toll will be nearer 100,000.

The Queen is to make a “personal donation” to help Japan and New Zealand after their quakes, Buckingham Palace said.