Swallows nesting at the Sellafield nuclear complex in Cumbria have been contaminated with low levels of radioactivity from the site, according to the Environment Agency.

Monitoring by the site's operator, Sellafield Limited, has found traces of radioactivity in the birds' droppings. This is thought to come from the insects they have consumed around the storage ponds for radioactive waste .

Investigations have been launched by the agency, the government's Office for Nuclear Regulation (ONR) and Sellafield, while measures are being taken to try and prevent the contamination from recurring. According to Sellafield, the levels of radioactivity discovered were very low and there was "no threat" to public health.

"There is no direct pathway for exposure to members of the public," said a Sellafield spokesman. "Measures are in place to reduce the possibility of birds gaining access to facilities."

Nutrients and nesting materials are being removed to try and make the site less attractive for birds' breeding. "Possible problems are reduced by applying the principle of good housekeeping and the incorporation of bird exclusion measures," the spokesman added.

Up to 30 swallows had been surveyed. "There was insufficient radioactivity in the guano to require personal protection beyond that required for hygiene purposes," he said. Though it was difficult to be sure, contaminated mosquitoes were "a possible pathway".

Anti-nuclear groups pointed out that swallows fly long distances. "These much-loved and now radioactive birds and their offspring will unwittingly be carrying a toxic message from Sellafield when they migrate back to southern Africa at the end of the summer," said Martin Forwood from Cumbrians Opposed to a Radioactive Environment.

"Given the history of bird contamination at Sellafield, it is quite extraordinary that it's been allowed to happen again. They have failed to deal with this problem for years, and should have covered up their waste ponds long ago."

More than 150 pigeons were found to be contaminated with Sellafield's radioactivity in 1998, and contaminated gulls were culled in 2010. There have also been problems with radioactive fruit flies and gnats at the Hanford nuclear plant in Washington State, US.

There are at least two open-air ponds at Sellafield, both dating back to the 1950s. They store radioactive waste from early nuclear reactors and may generate warmth that attracts insects.

Sellafield realised that birds could become contaminated with low levels of radioactivity as a result of historic operations at the plant. "Cleaning up the legacy buildings on the site will remove the source of contamination and remains a priority activity," said the site spokesman.

The contamination was reported to a sub-committee of the West Cumbria Sites Stakeholder Group last week by the Environment Agency. In an email yesterday to Forwood, Michael Ainsworth, one of the agency's nuclear regulators, confirmed that the "contamination of nesting sites" had been identified by Sellafield.

The agency was now awaiting a report from the site, he said. "They are in the process of carrying out an investigation into this issue and looking at ways of excluding birds from their nesting sites and breaking the life cycle of the mosquitoes – their food source." An ONR spokesman said: "Sellafield Limited is aware of the potential spread of radioactive material from the site by wildlife and, with the assistance of specialist wildlife control contractors, it has been taking steps over a number of years to address this matter."

He added: "Sellafield Limited's ongoing review indicates that the amount of radioactive material that may be distributed in this manner is unlikely to pose a threat to public health. We are keeping the situation under review and the matter will be followed up during our ongoing interactions with Sellafield Limited."