In one example, a pack of chicken nuggets was labelled with the salt content for a serving size of just one nugget.
Other examples include breakfast cereals with a portion size of 30g and ready meal curries with a size of 175g, which are lower than usual for these types of products.
The study was carried out by the Local Authorities Coordinators of Regulatory Services. It found that while the salt content of the foods tested has reduced by 10.9% since May 2005, fewer than half of the foods had achieved the specific salt reduction targets for 2010, as identified by the Food Standards Agency.
It also revealed that more than one in five noodle-type snacks, popular with children and young people, contain more than 4g of salt, which is two-thirds of the recommended daily consumption for an adult.
The study was conducted by environmental health officers from 60 local councils across the UK, who collected 831 samples of processed food from supermarkets.
Geoffrey Theobald, chairman of Lacors, said: "There is concern that customers are being hoodwinked and misled by some manufacturers who are deliberately quoting unreasonably small portion sizes on their packaging to mask the true salt content of their products. This is worrying as labels provide the main source of information for consumers wanting a healthy and balanced diet."