Food safety officials were last night trying to establish how 23 tonnes of organic free-range chicken sold from stores around Britain and Ireland contained a cancer-causing chemical banned in livestock since 1995.

The chicken, sold both as whole birds and in pieces, came from a farm in Northern Ireland, distributed under the brandnames Tesco, Waitrose, Morrisons, and Moy Park.

The chicken had all passed sell-by dates but the Food Standards Agency yesterday advised people not to eat any products left in their fridge. Routine checks in Northern Ireland revealed traces of nitrofurans, a veterinary medicine not to be used in food-producing animals.

Officials said the risk from eating an affected chicken was low, with concerns focused on long-term use. But it was "not acceptable" that traces of the drug had been found. Moy Park, a food processing firm, is cooperating with officials to find the source.