Three million eggs eaten every day could contain residues of a potentially harmful antibiotic, according to the Soil Association, the UK's main promoter of organic food and farming.

The drug, lasalocid, is an antibiotic which can legally be used in the feed of chickens sold for meat, turkeys, pheasants and quail. Lasalocid is contained in ready-made feed sold to farmers for these purposes. However the Soil Association said it is not licensed for use in feed for chickens that lay eggs.

The Soil Association warned that babies could be at risk, along with people who eat a large number of eggs, such as those on the Atkins' diet, and people with certain heart conditions.

According to government figures seen by the Soil Association, residues are increasingly being found in chickens' eggs. In 1999, only 1% of egg samples contained residues of lasalocid. This rose to over 12% in 2003 and one sample showed the highest level of contamination ever recorded (3,450 parts per billion).

"Past government assurances that residues of lasalocid pose no risk to health are shown to be based on false calculations which underestimate the consumption of egg, especially by children and take no account of toxic metabolites of lasalocid in food," the Soil Association said.

"Eggs contain beneficial nutrients and it is important that people who currently eat eggs continue to do so. But until this drug is banned those most at risk should limit the amount of egg eaten on any one day and consider buying organic eggs since these have to be the safest option," said Richard Young, the Soil Association's policy advisor.