Radiation levels at Japan's Fukushima nuclear plant are 18 times higher than previously thought, according to officials.
The news follows last week's announcement that 300 tonnes of toxic water had seeped into the ground from one of the site's 1,000 giant containers before anyone had noticed.
The Tokyo Electric Power Company (Tepco), which runs the facility, has now said the readings near that tank showed radiation strong enough to kill someone exposed to if for four hours.
The leak was categorised as a Level 3 event, the most serious category since the 2011 meltdown, caused by that March's earthquake and tsunami which killed thousands.
According to the BBC, Tepco made the initial readings using equipment that could only read emmissions of up to 100 millisieverts an hour, and got a reading of 100 millisieverts an hour.
A second recording, using more sensitive kit, showed 1,800 millisieverts an hour.
Japanese Prime Minister Shinzo Abe has promised his government will do more to prevent radioactive leaks.
He said: "The accident in Fukushima cannot be left entirely to Tokyo Electric Power. There is a need for the government to play a role with a sense of urgency, including taking measures to deal with the waste water."