© The Associated PressCollecting water: The Self-Defense Forces's helicopter scoops seawater on Japan's northeast coast en route to the Fukushima plant
Japan has 48 hours to bring its rapidly escalating nuclear crisis under control before it faces a catastrophe "worse than Chernobyl", it was claimed last night.

Nuclear safety officials in France said they were "pessimistic" about whether engineers could prevent a meltdown at the Fukushima power plant after a pool containing spent fuel rods overheated and boiled dry.

Last night radiation levels were "extremely high" in the stricken building, which was breached by an earlier explosion, meaning that radiation could now escape into the atmosphere.

Tokyo Electric, the owners of the plant, said five workers had been killed at the site, two were missing and 21 had been injured.

Last night, a US nuclear safety chief said the Japanese government had failed to acknowledge the full seriousness of the situation at the Fukushima plant and that warnings to citizens had been insufficient and understated.

Gregory Jaczko, the chairman of the US Nuclear Regulatory Commission, warned that if "extremely high" radiation levels increased it would become impossible for workers to continue to take corrective measures at the plant as "the doses they could experience would potentially be lethal doses in a very short period of time".

As Japan resorted to increasingly desperate measures - including dumping water on the site from helicopters - there were accusations that the situation was now "out of control".

The Foreign Office responded to the latest developments by advising all British citizens to leave Tokyo - which is 140 miles south of the plant - and the whole of northern Japan. The EU has even urged member states to check Japanese food imports for radioactivity.

Emperor Akihito made a rare address to the nation, urging the Japanese to pull together, but hinted at his own fears for the nuclear crisis saying: "I hope things will not get worse." In London, the FTSE - 100 share index slumped as news of the latest emergency emerged, closing 1.7 per cent down.

The official death toll from last Friday's earthquake and tsunami stood at 4,314 last night, with another 8,606 listed as missing.

Thousands of people still waiting for food aid in the most remote areas of the disaster zone endured fresh misery yesterday as heavy snow began to fall across northern Japan. But all eyes were on the Fukushima Daiichi nuclear plant as Japanese authorities admitted concerns over rising temperatures in three pools containing spent fuel rods.

A failure of the cooling system that has crippled the entire plant led to water boiling in the No4 pool. Last night the US Nuclear Regulatory Commission (USNRC) said there was no water left in the pool, resulting in "extremely high" radiation levels. An earlier fire and explosion in the No 4 reactor building is thought to have breached the protective walls around the pool. A statement from the USNRC said: "We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures."

If the water has gone, Mr Jaczko warned, there is nothing to stop the fuel rods from getting hotter and ultimately melting down. The outer shell of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.

Jeremy Browne, the Foreign Office minister, said: "There is clearly an evolving situation and things are clearly getting worse at the Fukushima nuclear plant."

Attempts to cool the site by dumping sea water from helicopters had to be aborted at one stage because of dangerous radiation levels in the air above the plant. A police water cannon was brought in to help blast water into the overheating reactors and pools, but there were warnings that it may be too late to prevent a disaster. Thierry Charles, a safety official at France's Institute for Radiological Protection and Nuclear Safety (IRSN), said: "The next 48 hours will be decisive. I am pessimistic because, since Sunday, I have seen that almost none of the solutions has worked." He described the situation as "a major risk", but added: "All is not lost."

Asked about the maximum possible amount of radioactive release, he said "it would be in the same range as Chernobyl".

The incident at the Chernobyl nuclear power plant in Ukraine on April 26 1986 is estimated to have caused 57 direct deaths, with some 4,000 additional deaths from cancer.

Francois Baroin, a French government spokesman, went further, saying: "In the worst of cases, it could have an impact worse than Chernobyl." He added: "They have visibly lost the essential of control."

Malcolm Grimston, a British nuclear expert at the Chatham House think tank, played down suggestions of an impending disaster, saying Fukushima was not like Chernobyl.

"We're nearly five days after the fission process was stopped, the levels of radioactive iodine will only be about two - thirds of where they were at the start, some of the other, very short - lived, very radioactive material will be gone altogether by now," he said.

Earlier, Nathalie Kosciusko - Morizet, France's ecology minister, had said that "the worst scenario is possible and even probable". At one point, radiation levels at the plant rose to such dangerous levels that all workers were evacuated from the site. A 180 - strong team was later allowed back to continue attempts to cool the fuel rods, but the government raised the maximum allowable radiation exposure for workers from 100 millisieverts per year to 250, which it said was "unavoidable due to the circumstances".

Yukiya Amano, the International Atomic Energy Agency's director general, said the situation was "very serious". He is flying to Japan today for a first - hand briefing on the crisis.