They also measured caesium in terrestrial vegetation and in lake water, and found that its effective ecological half-life rose from between one and four years in the first five years after Chernobyl to between six and 30 years recently.
Levels of radioactivity from the Chernobyl explosion in 1986 remain unexpectedly high in some parts of northern Europe, researchers have found.
They say restrictions on some foods in both the United Kingdom and the former Soviet Union will have to remain in place for up to 50 years.
They found that the environment is not cleaning itself as fast as previously thought, and that radioactivity can be released to the soil again after it has been absorbed.
This unexpected result means that previous estimates of how long restrictions would be needed are proving wide of the mark.
The researchers' findings are published in Nature. They are from the UK and the Netherlands.
Increased half-life
They found from analysis of radioactivity in fish in Norway and Cumbria, in north west England, that levels of one element, caesium 137, were higher than expected.
They also measured caesium in terrestrial vegetation and in lake water, and found that its effective ecological half-life rose from between one and four years in the first five years after Chernobyl to between six and 30 years recently.
The team leader is Dr Jim Smith, of the Centre for Ecology and Hydrology, part of the UK's Natural Environment Research Council.
Dr Smith said: "During the first five years after Chernobyl, concentrations of radioactive caesium in most foodstuffs and water decreased by a factor of 10, but in the last few years they have changed very little.
"The environment is not cleaning itself of the pollution at the rate we previously thought.
"Generally, after a nuclear accident, ecosystems have a self-cleaning capacity, and caesium becomes immobilised in the soil.
Restoring balance
"Now, 14 years after Chernobyl, we have found that the caesium is not completely immobilised, but can be re-released to the ecosystem."
Dr Smith told BBC News Online this renewed leaking of radioactivity is not unprecedented.
"With many chemical processes there can be a back reaction, when the contamination diffuses out. Here the rate of absorption is slowing, and it's being matched by diffusion.
"It's like a physical principle. Diffusion happens because of a concentration gradient, which leads to a balance between the radioactivity in the water in the soil, and that absorbed into the soil itself.
"As that balance changes, the gradient levels out, and the difference between take-up and release alters."
The researchers say the contamination "represents only a small health risk to consumers".
Half century
But they say restrictions on the consumption of some affected foods will be needed for years ahead.
In 1986, UK Government scientists thought Cumbrian sheep should be kept out of the food chain for a matter of weeks.
Dr Smith and his colleagues say the restrictions may be needed for another 10 to 15 years, 100 times longer than originally estimated.
And forest berries, fungi and fish from parts of the former Soviet Union will remain restricted for another half century.