An investigation has established that the plastic exterior of more than one in every 100 raw chickens sold by Britain’s biggest retailers is harbouring a potentially infectious level of campylobacter.

Campylobacter is the UK’s leading cause of food poisoning with 485,000 recorded cases last year, according to Public Health England.

The Food Standards Agency estimates that the bug causes about 100 deaths a year. Previously the agency has described the risk of people becoming ill as a result of bacteria on the outside of packaging as “extremely unlikely”.

But food safety experts are now warning that hungry shoppers who eat snacks with their bare hands after picking up plastic packs of raw supermarket chicken are unwittingly putting themselves at risk.

Lisa Ackerley, an independent food hygiene expert, said: “I strongly suspect that many food poisoning cases are from cross contamination, which may involve the spread of bacteria from contaminated hands to mouth as well as via contaminated food.”

The routes of transmission could be much wider than people realise. “For example, people quite often get hungry in the supermarket and may buy snacks such as crisps to eat on the way home. But if they’ve picked up chicken with bacteria on the outside packaging and licked their fingers they could consume enough bacteria to become ill.”