Tokyo Electric Power Co. (TEPCO) said caesium equivalent to 254,000 becquerels per kilogramme -- or 2,540 times more than the government seafood limit -- was detected in a "murasoi" fish.

The fish, similar to rockfish, was caught at a port inside the Fukushima plant, a TEPCO spokesman said.

The Fukushima Daiichi nuclear plant was battered by a towering tsunami following a huge earthquake on March 11, 2011, causing reactor meltdowns which spewed radioactive contamination into the atmosphere.

Fishing around Fukushima was halted and the government banned beef, milk, mushrooms and vegetables from being produced in surrounding areas.