Health-conscious office-workers may plump for salads over sandwiches, which are perceived to be more calorific. But the survey of 20 pre-prepared salads found the average had 412 calories and 20.3 fat.

By contrast, a Big Mac has 490 calories and 24 grams of fat.

"Many of the salads we looked at were surprisingly high in calories and fat,” said editor Martyn Hocking. “Mainly this was because they had mayonnaise or creamy sauces. The ingredient lists showed these were often added generously."