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Radiation levels around Japan's Fukushima nuclear plant are 18 times higher than previously thought, Japanese authorities have warned.

Last week the plant's operator reported radioactive water had leaked from a storage tank into the ground. It now says readings taken near the leaking tank on Saturday showed radiation was high enough to prove lethal within four hours of exposure. The plant was crippled by the 2011 earthquake and tsunami. The Tokyo Electric Power Company (Tepco) had originally said the radiation emitted by the leaking water was around 100 milliseverts an hour.

However, the company said the equipment used to make that recording could only read measurements of up to 100 milliseverts.

The new recording, using a more sensitive device, showed a level of 1800 milliseverts an hour. The new reading will have direct implications for radiation doses received by workers who spent several days trying to stop the leak last week, the BBC's Rupert Wingfield-Hayes reports from Tokyo. In addition, Tepco says it has discovered a leak on another pipe emitting radiation levels of 230 milliseverts an hour.  The plant has seen a series of water leaks and power failures. The 2011 tsunami knocked out cooling systems to the reactors, three of which melted down.

The damage from the tsunami has necessitated the constant pumping of water to cool the reactors. This is believed to be the fourth major leak from storage tanks at Fukushima since 2011 and the worst so far in terms of volume. After the latest leak, the Japanese nuclear energy watchdog raised the incident level from one to three on the international scale that measures the severity of atomic accidents.

Experts have said the scale of water leakage may be worse than officials have admitted.