Official tests have confirmed the presence of radioactive waste from the Sellafield nuclear plant in farmed salmon.
Traces of radioactive waste were found
The Food Standards Agency (FSA) decided to conduct its own investigation into Technetium-99 (Tc-99) after a Greenpeace study found traces in samples of fish.
The environmental campaign group found very low levels Tc-99 in fresh and smoked salmon bought from Sainsbury's, Tesco, Asda, Safeway, Waitrose, and Marks & Spencer.
The FSA said the levels it found were below those reported in last month's Greenpeace report.
According to a spokesman, a person would need to eat more than 91 tonnes a year of fish containing Tc-99 at the amount it detected to reach the EU dose limit.
The FSA's tests were conducted in conjunction with the Scottish Environmental Protection Agency and involved 14 fish farms in waters off Scotland and one in Northern Ireland.
The findings detected traces of Tc-99 in fish at the Irish farm and four of the Scottish farms.
Dr Jon Bell, chief executive of the FSA, said: "The vast majority of the samples did not pick up anything.
"In those that did, the levels were very low and below a cause for human health.
"It is clearly not acceptable that something that could be avoided has ended up in the human food chain in this way."
The FSA Jean McSorley, nuclear campaign coordinator for Greenpeace, said: "Technetium from nuclear reprocessing shouldn't be in fish bought by consumers.
"The fact that we have had our results confirmed is good but the real problem is how do you stop the discharges.
"There is no immediate harm to health. However, we know from other instances that radioactive waste may accumulate in sea food."