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.
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 that 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.
It said consumers could avoid the risk of food poisoning by practising good food hygiene and cooking chicken thoroughly.
The watchdog also repeated advice not to wash raw chicken as it could splash the bacteria onto the sink, worktops or nearby dishes, increasing the risk of cross-contamination.
Campylobacter is the most common cause of food poisoning in the UK, according to the FSA. It is found mainly in poultry but also in red meat, unpasteurised milk and untreated water.
It was responsible for more than 371,000 estimated cases in England and Wales in 2009, resulting in more than 17,500 hospitalisations and 88 deaths.
Listeriosis, the foodborne illness caused by listeria, is relatively rare but listeria causes more deaths from food poisoning in the UK than any other foodborne bugs, with vulnerable groups at increased risk, the FSA said.
Between 2000 and 2009 the annual number of laboratory-confirmed cases of listeriosis more than doubled from 114 to 234, although this dropped in 2010 to 174.
Most people infected with listeria are hospitalised and approximately a third die.
A Which? study in February last year showed that 82% of the public want controls in place throughout the supply chain to prevent chickens becoming infected rather than trying to deal with contamination at the end of the process.
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."
British Poultry Council chief executive Peter Bradnock said: "This report makes clear that chicken is a safe and healthy product when properly cooked.
"These welcome findings show a big reduction in campylobacter presence on chicken, demonstrating the effectiveness of the biosecurity measures being taken by producers and processors against this naturally occurring bacteria. The British poultry industry is committed to working with consumer groups, government and retailers to ensure chicken is safe and healthy, and remains Britain's favourite meat."