The Co-operative has emerged as the surprise loser in a major six-monthly customer service survey.

The Institute of Customer Service asked a panel of 25,000 shoppers to score the major food retailers. They voted Waitrose the best food retailer, with a score of 87.1%, but the Co-operative Group in last place with 74.3%, trailing Waitrose by a full 14 percentage points.

The scores of the big four were much closer: Asda had the highest rating, with 80.8%, Tesco the lowest on 77.5%.

Customers judged the stores on professionalism, efficiency, ease of shopping and timeliness. The food sector narrowly beat non-food, with an average score of 78.7% versus 78.4%.

The institute said good service helped retailers grow profits and increased the likelihood of repeat shopping: customers who were 'very satisfied' were three times more likely to return than a merely 'satisfied' shopper.

"We know businesses and individuals have less money to spend and are more careful where to spend it," said ICS chief executive Jo Causon. "But research shows taking customer service seriously can produce 24% more profit. Even a 5% jump in customer loyalty can boost profits by between 25% and 85%."