Radiation near a tank holding highly contaminated water at Japan's crippled Fukushima nuclear plant has spiked 18-fold, the plant's operator says. 

Radiation of 1,800 millisieverts per hour - enough to kill an exposed person in four hours - was detected near the bottom of one storage tank yesterday according to the Tokyo Electric Power Co (Tepco). 

Readings on Aug 22 measured radiation of 100 millisieverts per hour at the same tank.

Japan's legal limits for nuclear plant workers' exposure during normal hours is just 50 millisieverts 

Last month, Tepco revealed that water from the tank was leaking. Japan's chief cabinet secretary has described as the situation as "deplorable".

The Japanese nuclear regulator has raised the leak from a level 1 "anomaly" to a level 3 "serious incident" on the international scale for radiation releases.

The Fukushima plant was devastated by a tsunami on 11 March 2011 causing meltdowns at three reactors, radioactive contamination of the air, sea and food and the evacuation of 160,000 people.

It sparked the worst nuclear crisis since Chernobyl in Ukraine in the former USSR in 1986.

While there were no new leaks found at the tank, a Tepco spokesman said another leak had been detected from a pipe connecting two other tanks nearby.

"We have not confirmed fresh leakage from the tank and water levels inside the tank have not changed," the Tepco spokesman said. "We are investigating the cause."

Tepco said the radiation measured was beta rays, which would be easier to protect against than gamma rays.

The Tepco spokesman also said the higher level of radiation from the latest reading was partly because investigators had used a measuring instrument capable of registering greater amounts of radiation.

Instruments used previously had only been capable of measuring radiation up to 100 millisieverts, but the new instruments were able to measure up to 10,000 millisieverts.

With no one seeming to know how to bring the crisis to an end, Tepco said last week it would invite foreign decommissioning experts to advise it on how to deal with the highly radioactive water leaking from the site.