The consumer group says that 17 per cent were contaminated with listeria.

Of the 192 samples tested by Which? 1.5 per cent also tested positive for salmonella.

Which? tested whole chickens and chicken portions from nine supermarkets in March 2012.

Of the 192 samples:

• one in five (18 per cent) were contaminated with campylobacter
• 17 per cent were contaminated with listeria, with four per cent containing levels of listeria classed as high by the Food Standards Agency (FSA)
• 1.5 per cent tested positive for salmonella.

The Which? study was a snapshot, testing chicken samples from all the major retailers – Aldi, Asda, The Co-operative, Lidl, Marks & Spencer, Morrisons, Sainsbury's, Tesco and Waitrose.

The chicken samples were bought on two days in different locations. Bacterial contamination was found in samples from every retailer tested.

Although not a directly comparable test, the Which? results indicate an improvement on 2009, when the FSA found that 65 per cent of fresh chickens it tested were contaminated with campylobacter at the point of sale.

Which? executive director Richard Lloyd said: "While the situation is improving, it is 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 food chain."

In response to the Which? report on supermarket chicken, Peter Bradnock, Chief Executive of the British Poultry Council, said: "The BPC notes the Which? report, which makes clear that chicken is a safe and healthy product when properly cooked.

"This new survey shows 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 which is present is all live animals.

"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."