Millions of eggs each day could be contaminated with a toxic drug, scientists have warned.
Scientists tested eggs for the drug
Traces of lasolocid, which is used to tackle a parasitic stomach illness in chickens, were found in 4.8% of eggs tested in 2002.
That could mean the drug was present in around 473m eggs eaten last year.
But the Veterinary Residues Committee, which carried out the tests, said its findings did not constitute a risk to human health.
Chickens on factory farms are routinely given lasolocid in their feed in a bid to prevent them being infected with the stomach illness.
They are supposed to be taken off the feed at least nine days before they are slaughtered to allow residues to leave their system.
These rules do not apply to hens farmed for eggs, who are not supposed to be given the medicated feed at all.
A spokesman for the Veterinary Residues Committee, which published its annual report on Monday, said: "The VRC noted with concern that the feed additive could be found in poultry products.
"But they don't consider the residues found to be a risk to human health."
However, he added that the committee had said it would like to see levels at "nil", if possible.
The committee say it is not being given to chickens by farmers, but is getting into the feed during production.
Animal medicine inspectors will now be sent out to feed factories to check there is no contamination between feed produced for chickens which are to be slaughtered and those which lay eggs.
The Soil Association says tests in animals show the drug, given to combat the coccidia parasite, was highly toxic.
Safe levels for humans are worked out on the basis of animal test results.
But Richard Young, of the association, said: "We want this drug to be banned.
"It's impossible to prove it's a danger to human health because it hasn't been tested. But we're dealing with a dangerous drug which is going into human food on a regular basis, and sometimes at very high levels."