Tests were carried out on salmon

The tests were commissioned by environmental campaign group Greenpeace, which blamed discharges from the Sellafield nuclear plant in northern England.

However, the Food Standards Agency (FSA) said the findings were well below permitted levels.

Dr Andrew Wadge, the FSA's director of food safety, said: "Even at the maximum concentrations found by Greenpeace, a person would have to eat 700 portions of salmon a day for a year to reach the annual permitted EU radiation dose.

"This means consumers have no need to be alarmed.

"Low levels of technetium-99 (Tc-99) are found routinely in lobster, shellfish and other fish from waters around Sellafield."

Radioactive waste

The tests were conducted by Greenpeace's research laboratory at Exeter University.

It found very low levels of technetium-99 in fresh and smoked salmon bought from Sainsbury's, Tesco, Asda, Safeway, Waitrose, and Marks and Spencer.

Greenpeace nuclear campaign co-ordinator Jean McSorley said: "Sellafield has been discharging radioactive waste for the last five decades.

"Technetium has become a particular worry because Sellafield started discharging a backlog of this material into the Irish Sea."

A person would have to eat 700 portions of salmon a day for a year to reach the annual permitted EU radiation dose Dr Andrew Wadge Food Standards Agency

A spokesperson for Sainsbury's said it would investigate when it saw the report, stressing: "Our customers' food safety is our first priority."

Marks and Spencer said: "We share concerns about the unnecessary pollution of our oceans and therefore welcome the government's action on reducing discharge levels from Sellafield.

"We also support the commitment to additional research to allow the discharge levels of Tc-99 to be reduced more quickly."

Tesco stressed that the salmon sold in its stores was safe to eat.

"We have conducted safety tests on Scottish farmed salmon to check for traces of Tc-99," said a spokesperson.

"The results show that the traces were well below the safety limits set by government."