Nearly three-quarters of fresh shop-bought chickens have tested positive for food poisoning bug campylobacter and for the second year running Asda is the worst offender, a survey has revealed.
Some 19% of the chickens in the year-long Food Standards Agency survey contained the highest rate of the bug, which affects an estimated 280,000 people a year.
Campylobacter contamination can cause intestinal infections
The testing, which ended in February, looked at the prevalence and levels of campylobacter contamination on fresh whole chilled chickens and their packaging, and revealed that Asda was the worst offender - a position it also held last year.
The chain had a higher-than-average incidence of chicken contaminated at the highest level, while Tesco was the only supermarket to fall below the industry average.
The FSA's cumulative results from the first three quarters of the survey show that the overall rate of contamination has stalled at 73% since February but risen from 70% in November and 59% in August.
The full survey results which show that Asda was the worst offender
A second year of testing will begin this summer to measure the impact of interventions being introduced by the industry to tackle contamination.
More than 4,000 samples of fresh whole chickens and packaging from large UK retail outlets and smaller independent stores and butchers were tested as part of the survey.
While none of the major retailers hit industry targets during the FSA investigation, the food watchdog said internal case studies by Marks & Spencer, Morrisons, the Co-op and Waitrose based on more recent samples found a fall in contamination levels.
FSA director of policy Steve Wearne said: "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.
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"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.
'"We are going to run this survey for a second year and will again look at campylobacter levels on chickens at retail sale. I hope that we will be able to see the results from the actions taken by the four retailers mentioned above and others come through and produce much lower figures for the incidence of campylobacter on the chicken we buy."
Asda had the highest rate of contamination in its fresh whole chickens at 80%, followed by Co-Op with 78%, Morrison's with 76%, Waitrose with 74%, and Tesco and Marks and Spencer with 67%.
Individual figures for Lidl, Aldi and Iceland are not individually available because their market share was not considered large enough, based on 2010 retail figures.
Their results were instead reported together with those from independent butchers and convenience stores, reaching 73%.