Environmental group Greenpeace warned Thursday?marine life?radiation levels are 50 times higher than official limits in the ocean off Japan's stricken Fukushima nuclear plant.

A Greenpeace crew tests waters off Japan for radiation contamination earlier this month. [Greenpeace]

New data from?Greenpeace's radiation monitoring showed seaweed radiation levels 50 times higher than official limits, raising serious concerns about continued long-term risks to people and the environment from contaminated seawater.

Earlier this month, Greenpeace radiation monitoring teams??collected samples of marine life including fish, shellfish and seaweed outside Japan's 12-mile territorial waters and along the Fukushima coast.

Detailed analysis by accredited laboratories in France and Belgium found high levels of radioactive iodine contamination and?caesium in the samples.

"Our data show that significant amounts of contamination continue to spread over great distances from the Fukushima nuclear plant", said Jan Vande Putte, Greenpeace Radiation Expert.

"Despite what the authorities are claiming, radioactive hazards are not decreasing through dilution or dispersion of materials, but the radioactivity is instead accumulating in marine life."

Mr Vande Putte accused Japan of doing too little to measure and share data on marine life contamination and said: "Japan's government is mistaken in assuming that an absence of data means there is no problem."

Most fish and shellfish sampled by Greenpeace were found to contain levels of radioactivity above legal limits for food contamination. This is just one of the multiple, chronic sources of radiation exposure to people living in the greater Fukushima area. In April, the authorities raised regulatory limits for levels of radiation exposure twentyfold to 20 milliSievert per year for all people – including children.

"Ongoing contamination from the Fukushima crisis means fishermen could be at additional risk from handling fishing nets that have come in contact with radioactive sediment (6), hemp materials such as rope, which absorb radioactive materials, and as our research shows, radioactivity in fish and seaweed collected along Fukushima's coast," said Wakao Hanaoka, Greenpeace Japan Oceans Campaigner.

"Fishermen, their communities and consumers desperately need information on how radioactivity affects their lives, livelihoods and the ecosystems they rely on, and especially how they can protect themselves and their families from further contamination."