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Which? executive director Richard Lloyd said: "While the situation is improving, it is still unacceptable that one in five chickens we tested were found to be contaminated with campylobacter.
"We want to see the risk of contamination minimised at every stage of production, because for far too long consumers have been expected to clean up mistakes made earlier in the supply chain."
The watchdog tested 192 samples of whole chickens and chicken portions - standard, free range and organic and all reared in the UK - from Aldi, Asda, The Co-operative, Lidl, Marks & Spencer, Morrisons, Sainsbury's, Tesco and Waitrose in March.
Bacterial contamination was found in samples from each of the retailers.
Which? stressed the study was a "snapshot" as it tested each retailer on two days in different locations, and was therefore unable to definitively conclude that chicken from one supermarket was better than that from another.
However the results indicated an improvement on 2009 when the FSA found that 65% of fresh chickens it tested were contaminated with campylobacter at the point of sale.
One in five supermarket chickens is contaminated with the food poisoning bacteria campylobacter, an investigation has found.
The study of chicken samples from nine supermarkets by the Which? consumer group found 18% were contaminated with campylobacter and 17% were contaminated with listeria, with 4% containing levels of the latter classed as "high" by the Food Standards Agency (FSA).
Salmonella was present in 1.5% of samples.