Seawater outside the crippled nuclear power plant in north-eastern Japan was found to contain 3,335 times the usual amount of radioactive iodine - the highest rate yet and a sign that more contaminated water was making its way into the ocean, officials said today.

The amount of iodine-131 found offshore 300 yards south of the coastal Fukushima Dai-ichi power plant did not pose an immediate threat to human health, but was a "concern", said Hidehiko Nishiyama, of the Nuclear and Industrial Safety Agency. He said there was no fishing in the area.

"We will nail down the cause, and will do our utmost to prevent it from rising further," Mr Nishiyama said.

The power plant has been leaking radiation since the March 11 tsunami slammed into Japan's north east, knocking out power and back-up systems crucial to keeping temperatures down inside the plant's reactors.

Residents within 12 miles have been evacuated, while those up to 19 miles have been urged to leave as radiation has made its way into vegetables, raw milk and water. Last week tap water as far away as Tokyo, 140 miles to the south, contained levels of cancer-causing iodine-131 considered unsafe for infants.

The latest findings - based on a sample taken yesterday - highlight the urgency of stabilising the power plant. The mission has been fraught with setbacks, as emergency crews have dealt with fires, explosions and radiation scares in the frantic bid to prevent a complete meltdown.

The government acknowledged yesterday that its safeguards had been insufficient to protect the plant against the magnitude-9.0 earthquake and tsunami.

"Our preparedness was not sufficient," chief Cabinet secretary Yukio Edano said. "When the current crisis is over, we must examine the accident closely and thoroughly review" the safety standards.

An Associated Press investigation found that Tokyo Electric Power Co (Tepco) officials had dismissed scientific evidence and geological history that indicated that a massive earthquake - and subsequent tsunami - was far more likely than they believed.

That left the complex with nowhere near enough protection against the tsunami.

Highly toxic plutonium was the latest contaminant found seeping into the soil outside the plant, Tepco said. Safety officials said the amounts did not pose a risk to humans, but the finding supports suspicions that dangerously radioactive water is leaking from damaged nuclear fuel rods.

"The situation is very grave," Mr Edano said.

Workers succeeded last week in reconnecting some parts of the plant to the power grid. But as they pumped in water to cool the reactors and nuclear fuel, they discovered numerous pools of radioactive water, including in the basements of several buildings and in trenches outside.

The contaminated water has been emitting many times the amount of radiation that the government considers safe for workers. It must be pumped out before electricity can be restored and the regular cooling systems powered up.

That has left officials struggling with two crucial but contradictory efforts: pumping in water to keep the fuel rods cool and pumping out contaminated water.

Officials are hoping tanks at the complex will be able to hold the water, or that new tanks can be trucked in. The Nuclear Safety Commission said other possibilities included digging a storage pit for the contaminated water, recycling it back into the reactors or even pumping it to an offshore tanker.

Yesterday three workers trying to connect a pump outside the Unit 3 reactor were splashed by water that gushed from a pipe. Though they wore suits meant to be waterproof and protect against high levels of radiation, Mr Nishiyama said the men were soaked to their underwear.

They quickly washed it off and were not injured, officials said.

Last week, two workers were hospitalised with burns after they waded into highly radioactive water that reached their knees while wearing ankle-high protective boots. They have been treated and released.

Nikkei, Japan's top business newspaper, called it "outrageous" that Tepco had been slow to release information about trenches outside the reactors filled with contaminated water, one just a few inches (10 centimetres) from overflowing.

Tepco's shares plunged nearly 20% yesterday. Its share price has nosedived a staggering 73% since the tsunami.

Prime minister Naoto Kan reiterated in a speech to parliament that Japan was grappling with its worst problems since the Second World War.

More than 11,000 bodies have been recovered, but officials say the final death toll is expected to exceed 18,000. Hundreds of thousands of people remain homeless, their homes and livelihoods destroyed. Damage could amount to £194 billion - the most expensive natural disaster on record.

"This quake, tsunami and the nuclear accident are the biggest crises for Japan" in decades, Mr Kan said. He said the crises remained unpredictable, but added: "We will continue to handle it in a state of maximum alert."

Mr Kan has faced increasing criticism from opposition MPs over the handling of a nuclear disaster stretching into a third week.

Meanwhile, Tepco's president has been admitted to hospital with high blood pressure and dizziness, the company said today.

Masataka Shimizu has not been seen in public since several days after the earthquake and tsunami. Tepco spokesman Takehiko Yamanaka said Mr Shimizu had been staying at the company's headquarters.