Some supermarket salads contain more fat than a Big Mac and fries, it was revealed today.

According to consumer group Which?, leafy lunches stocked at Morrisons and Asda are among the least healthy options.

The Smedleys Atlantic Prawn Marie Rose Salad, sold at a number of supermarkets including Morrisons, contains 855 calories and 66.3g fat - 70 per cent of the fat a man should eat in a day.

In contrast, a Big Mac and medium portion of fries contains 820 calories and 40g of fat.

Another "unhealthy" pre-packed salad was the Asda Chicken Caesar Pasta Salad which contained 41g of fat - again more than a Big Mac and fries and nearly as much as six Cadbury's Creme eggs.

Almost a quarter of this salad was made up of high-calorie sauce - 13 per cent mayonnaise and 10 per cent Caesar dressing.

Creamy sauces were often the reason why salads were so surprisingly high in calories and fat, the report found.

Mayonnaise dressing was the second highest ingredient in an M&S Pasta with Tomato & Basil Chicken salad which had 760 calories and 46g fat.

Sainsbury's Tomato & Basil Chicken salad was also revealed to be comparatively high in fat.

The report, which highlighted differences in the labelling of salads, said it was sometimes difficult for consumers to identify the fat content of a meal.

Findings showed the Tesco Tuna Layered Salad only listed calories per half pack rather than showing the 550 calories and 41g fat found in a full salad.

Martyn Hocking, of Which?, said: "If you thought your high-street salad was healthy, you could be in for a surprise.

"Which? has found that there were large differences between the amount of fat, saturated fat, salt and calories in pre-packaged salads.

"This latest research backs up what we've been saying for ages - a clear, consistent labelling scheme is important to help people spot how much fat, sugar and salt is in the food they're buying."

But Morrisons termed the report "misleading" and said it misrepresented the company.

A spokesman said: "Morrisons sells around 1.5 million salads each week, and this particular branded line accounts for less than one tenth of one per cent of those sales.

"The comparison is absurd, Which? Is weighing up a mayonnaise based product against a leafy salad. It's not comparing like with like."

A spokesman for Asda added: "We are the only supermarket that gives customers the benefit of traffic light colours, guideline daily amounts, grams of nutrients per portion and the words 'high', 'medium' and 'low' on our products, the very labelling system that Which? is calling for and has recommended by the Food Standards Agency.

"Unlike other retailers, our customers can see at a glance which of our wide range of salads are the healthiest and which are a bit more of an indulgence."

And Sainsbury's insisted it offered "a range of salads" to cater for a broad spectrum of customers.

A spokesman said: "The salad highlighted by Which? is clearly labelled red for fat but is also labelled green for saturates as it contains only 2.1g of saturated fat as a result of the work we have done to reduce saturate levels across all of our own label ranges. It is also labelled green for salt and sugars."

Which? sampled 20 pre-packed salads on the high street to compile its report.

Of these, it found several "healthy options" including Sainsbury's Rainbow Salad which contains soya beans and lentils and Sainsbury's Thai Chicken Noodle is low in fat, salt and sat fat.

Both salads came with dressing in a separate container.

Which? is calling for food companies to adopt one type of label so shoppers can clearly identify levels of fat, sugar and salt.