Tests by the UK Food Standards Agency have confirmed the presence of radioactive waste from the Sellafield nuclear plant in salmon farmed in Northern Ireland.
The Food Standards Agency (FSA) decided to conduct its own investigation into Technetium-99 (Tc-99) following the results of a study commissioned by Greenpeace. The environmental campaign group found very low levels of Tc-99 in fresh and smoked salmon bought from Sainsbury's, Tesco, Asda, Safeway, Waitrose and Marks & Spencer.
The FSA's tests were conducted in conjunction with the Scottish Environmental Protection Agency. They involved 14 fish farms in waters off Scotland and one in Northern Ireland.
The findings, which have yet to be published, 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 said the levels it found were below those reported in the Greenpeace report last month.
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.
Ms Jean McSorley, nuclear campaign co-ordinator 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 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."
Last month the British government asked British Nuclear Fuels Ltd to impose a nine-month ban on emissions of low-level Technetium from Sellafield.
The Irish Government has put pressure on Britain to close the Cumbrian plant because of fears of the risk of radioactive pollution spreading across the Irish Sea. Norway has also voiced concern because it has one of the world's largest salmon farming industries.