Sellafield nuclear power plant has played down fears of a radioactive leak after iodine-129 was found in milk on a nearby farm.

Sellafield, based in Cumbria, said: “Any presence of iodine-129 in milk could be down to either historic or existing discharges from the site.”

Asked what levels would be dangerous to humans, spokeswoman Ali McKibbin said: “At the levels we have monitored a person would have to drink 75,000 litres of milk to receive the dose limit for members of the public.”

Ms McKibbin went on to try and justify why it is difficult to clarify where and when the contamination could have occurred despite the discovery in December 2007 and January 2008 by saying: “A possibility is that a cow ate a contaminated particle.

“But iodine-129 has a half-life of approximately 15.7m years which makes it hard to pinpoint the exact time a radiation leak could have occurred.”

Sellafield conducts routine checks every month and the Environment Agency felt that the incident was “just a blip, there is no great concern”.