Asda chickens are the most likely to be contaminated with the food poisoning bug campylobacter, according to figures published by the Food Standards Agency.
Campylobacter is found mainly in raw poultry, and is the biggest cause of food poisoning in the UK. It causes nausea, diarrhoea and vomiting, as well as crampy stomach pains and a high temperature. While most people recover within a week, it can lead to severe dehydration, which in some cases can even be fatal.
But this bug, says the FSA, is found in the vast majority of the UK's fresh chickens. Asda birds are most likely to be contaminated, with 80.4% of the supermarket's fresh chickens testing positive for the bug.
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The Co-op was next, with 78.1%, followed by Morrison's with 75.8%. The lowest rates of contamination were found in Tesco chickens,where 66.5% of samples tested positive.
Overall, 73% of chickens tested positive for the presence of campylobacter - 19% at the highest level of contamination. Seven percent of packaging tested positive, 0.1% at the highest level.
But while not one retailer has succeeded in hitting infection targets, the FSA says it finds the results from Marks & Spencer, Morrisons, the Co-op and Waitrose encouraging.
"They are making a real difference to public health, helping to cut down on the estimated 280,000 people who get ill from campylobacter each year. As we have always said, if you are prepared to work across the food chain to reduce the spread of this bug then you will get results," says Steve Wearne, FSA director of policy.
"I want to challenge those retailers who haven't yet demonstrated the impact that M&S, Morrisons, the Co-op and Waitrose are having on reducing campylobacter on chickens on their shelves. We expect all retailers and processors to be achieving the reductions we have seen in these retailers' figures – that's the only way we will meet the target we all signed up to."
In the meantime, consumers should be careful. Raw chicken should be covered and stored on the bottom shelf of the fridge, so that juices can't drip on to other foods and contaminate them.
Don't wash raw chicken: cooking will kill campylobacter, but washing chicken can spread it through splashing. Do, though, thoroughly wash and clean all utensils, chopping boards and surfaces used to prepare raw chicken - as well as your hands.
And make sure chicken is steaming hot all the way through before serving, with no pink meat and juices that run clear.
Campylobacter contamination rates (FSA)
Retailer % positive skin samples % positive pack samples
Asda 80.4 12.4
Co-op 78.1 4.9
Morrison's 75.8 11.2
Waitrose 73.8 9.7
Sainsbury's 69.7 4.9
M&S 67.1 2.9
Tesco 66.5 4.0