Asda, Morrisons and Iceland display junk food on four out of five checkouts, making them the 'worst offenders' for undermining parents' efforts to feed their children healthily, according to the Children's Food Campaign.
The Checkouts Checked Out report found that most supermarket branches and high street stores routinely promote unhealthy snacks at their tills, and that in many cases junk food is positioned at children's eye level.
The Co-op, Sainsbury's, Tesco and Waitrose were also criticised for displaying unhealthy food in queuing areas.
Sophie Durham, Children's Food Campaign spokesperson and co-author of the report, said: 'Impulse purchases at the checkout can add several hundred unplanned calories to a family shopping basket. Supermarkets claim to be responsible retailers, yet they continue to put their profits ahead of families' health.'
A spokeswoman for Sainsbury's said the supermarket was disappointed to have been singled out by the report.
'We are working with the government to help parents to make healthier choices. We are also launching a range of more than 300 healthier products, and have 50 promotions a week on fresh fruit and vegetables.'
A spokeswoman for Asda said: 'We thank the Children's Food Campaign for this report. We sell a wide range of products on our checkouts, from batteries to bags for life.'
Iceland declined to comment.