Greepeace says data from its radiation monitoring in the ocean off a stricken nuclear power plant in northeast Japan shows massive levels of contamination in seaweed and other marine life.
The Tokyo Electric Power Company (Tepco) has been trying to contain radiation from the Fukushima Daiichi plant crippled by a 9.0 magnitude earthquake and tsunami on 11 March.
Earlier in May, Tepco announced that the No 1 reactor most likely had a fuel meltdown shortly after the disaster and this week confirmed partial meltdowns at another two of the six reactors.
Greenpeace is warning that the environment and people are at long-term risk, the ABC reports.
After taking samples of fish, shellfish and seaweed collected in the Pacific off Fukushima, Greenpeace sent them for analysis at laboratories in Belgium and France.
The conservation group says results show that seaweed radiation levels are 50 times higher than official limits.
It says other marine samples also showed high levels of radioactive caesium and iodine.
Greenpeace says it proves that radioactivity is accumulating in marine life and not diluting, as claimed by Japanese authorities.