Eggs are monitored for traces of medicines

But the Soil Association disputes these figures.

"Government regulators have routinely provided misleading information in their public statements about the incidence of drug residues in chicken meat and eggs," says a new Soil Association report.

"They maintain that 99% of poultry meat and 97% of eggs are free of detectable residues. However, detailed analysis of the data on which their summaries are based suggests the actual levels could be up to 2,000% higher."

But Alastair Johnston of the British Veterinary Poultry Association (BVPA) says the UK is doing its best to minimise the use of anti-parasitic and antibiotic drugs.

He told BBC News Online: "As long as we aim to obey the withdrawal periods and maintain the standard of regulation concerning poultry drugs and antibiotics in the UK, then I think the poultry and eggs in the UK are the safest in the world."

Why are the drugs needed?

The Soil Association says that the use of such drugs "has its roots in intensive farming methods".

The report continues: "In this case, the specific problems are the unnatural feeding practices and unsanitary, overcrowded, moist, dark, confined conditions in which large numbers of chickens are kept - conditions under which most would undoubtedly perish without drugs to keep them alive until slaughter."

But Alastair Johnston, of the BVPA, says the health and welfare of birds would deteriorate if anti-parasitic and antibiotic products were not used, even in free-range conditions.

"You would use less of these products with free range chickens," he told BBC News Online. "But you would still need to use them to control disease."

"And would the public accept the consequence in terms of cost in the supermarket?" he adds.