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.
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