The findings are part of a survey by the health group Consensus Action on Salt and Health, which campaigns for health warnings on food.
It looked at the salt content of ready-made salads from high street stores and fast-food outlets, and found that one in five contained the same amount or more salt than a Big Mac and fries.
McDonald's was found to be the worst offender, with three of its salads having more than 5g of salt per serving, including the crispy chicken ranch salad with dressing and croutons. By comparison, a McDonald's Big Mac burger and large fries contains just over 3g of salt.
Other brands to feature in the top 10 are Pret a Manger, Iceland, KFC and the sandwich chain Eat.
The survey found that just under half of the salads surveyed had no labelling for salt levels, and that only 18 out of 81 had the salt per portion labelled on the packaging.
"Many people think salads, particularly in the summer, are a more healthy option," said Professor Graham MacGregor, chairman of CASH and professor of cardiovascular medicine at St George's Hospital in London.
"I don't understand how manufacturers can justify selling salads that contain over half, or in some cases all, of our daily recommended limit of salt in a single serving. No way can these salads be described as a healthy option -- one-fifth of them contain the same or more salt than a Big Mac and Fries.
When McDonald's launched its Salads Plus range, designed to bring more women into its restaurants, it faced criticism over the levels of fat in some of the dressings -- in some cases driving the fat content of the salad to a higher level of that for burgers and fries.
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