Just over three-quarters (76%) of fresh shop-bought chickens have tested positive for the food poisoning bug campylobacter in the latest Food Standards Agency (FSA) survey.
However the figure is down from the 83% of samples which tested positive at the same time last year, while the percentage of chickens testing positive for the highest levels of contamination has also dropped to 15% from 22% this time last year.
The FSA welcomed the "signs of progress" on the bug, which affects an estimated 280,000 people a year.
Only two retailers - The Co-operative and Waitrose - met the current industry target of selling no more than 7% of chickens with the highest levels of contamination.
The FSA said research had shown that reducing the proportion of birds in this category would have the biggest positive impact on public health.
Morrisons had the highest overall level of chickens testing positive for the bug at 86.2%, followed by Marks & Spencer (82.7%) and Sainsbury's (80.4%).
Morrisons also topped the table for chickens sold with the highest levels of contamination at 25.7%, followed by Asda (23.5%) and M&S (18.3%).
The first quarter results of the FSA's second survey, based on testing between July and September, looked at the prevalence and levels of campylobacter contamination on fresh, whole chilled chickens and their packaging.
The second year of testing is designed to measure the impact of interventions being introduced by the industry to tackle campylobacter.
The survey also found that 6% of packaging tested positive for the bug, and 0.3% tested positive at the highest band of contamination.
The FSA tested 1,032 samples of fresh whole chilled UK-produced chickens and packaging bought from large UK retail outlets and smaller independent stores and butchers.
It has been testing chickens for campylobacter since February last year and publishing the results as part of its campaign to encourage the whole industry to tackle the problem.
FSA director of policy Steve Wearne said: "It is good to see that some retailers are getting to grips with campylobacter. However, we want to see all of them pulling together to achieve real and lasting reductions.
"I am also pleased that we are starting to see retailers and processors being open with consumers about what they are doing to tackle the problem and about the impact their interventions are having on the chickens they are selling."