The environmental group Greenpeace says samples of vegetables taken around Fukushima showed radiation levels many times above legal limits.
It says radioactivity detected in two places was enough to expose people in a matter of weeks to the maximum yearly dose allowable.
Nuclear safety officials say radiation leaks are still small compared to the Chernobyl disaster, but Fukushima’s operators admit they could get worse.
“If the radiation leak is not stopped completely, then it is likely that the total cumulative radiation could eventually exceed Chernobyl,” said Junichi Matsumoto, a spokesman for the Tokyo Electric Power Company (Tepco).
Earlier Tepco’s chairman visited Fukushima prefecture and apologised for the ongoing problems.
The government has been taking steps to reassure people that fresh produce in the shops is safe.
At a promotional event in Tokyo, the fruit and vegetables on sale were from Iwaki, a city in an area that is home to nuclear power stations.
One tomato seller admitted he was alarmed:
“Level seven is the same as Chernobyl – it’s the first time I’ve felt a bit shocked,” he said.
There have been several contamination scares after high radiation levels were found in milk and vegetables.
But farmers were then given the green light to sell their produce after new tests found levels to be safe.
A senior government minister, Chief Cabinet Secretary Yukio Edano, was on hand to provide what he hoped would be a reassuring gesture as he chomped through a tomato.
He did not appear to be enjoying the experience, and a smile might have helped his cause.