Many pre-packaged supermarket salads are not as healthy as they seem, according to a new study.

Which? reveals that one – Smedleys' Atlantic prawn Marie Rose salad – had more calories than a McDonald's Big Mac and medium fries.

The ­consumer organisation looked at 20 pre-packaged salads from high street chains. Asda's chicken caesar pasta salad contained 43g of fat.

Labels were confusing and even ­baffling in some cases, Which? said. It is urging food companies to adopt one consistent labelling scheme so that shoppers can see at a glance levels of fat, sugar and salt.