The operator Tokyo Electric Power Co said Tuesday it had found 25,800 becquerels per kilogram of radioactive caesium in greenling, 258 times higher than the government safety standard.

Fishing in waters off the plant has been voluntarily restricted since the nuclear disaster at the plant, which went into meltdown after the March 2011 earthquake and tsunami.

Less than a month after the start of the disaster, Tokyo Electric dumped more than 11,000 tons of wastewater containing radioactive substances into the Pacific.

The previous record of radioactive contamination in fish was 18,700 becquerels per kilogram detected in cherry salmon caught in March, according to the Fisheries Agency.

Wakao Hanaoka, a Greenpeace Japan official, said the government now needs to carry out a full investigation of radioactive contamination in a wide range of sea areas off Fukushima, which has not been done yet.

The organization's surveys show higher levels of radioactive contamination were found in fish and seaweed sampled in areas further from the Fukushima plant.

Factors that affect the spread of contamination include ocean currents and seabed configuration, Hanaoka added.