Consumers are being misled about the salt content of some ready meals and other processed foods, according to a survey published yesterday.

The report accuses manufacturers of deliberately understating portion sizes on food labels, in an apparent effort to make the amount of salt in their products seem as low as possible. In one case a packet of chicken nuggets gave the salt content for a portion weighing 15g, the equivalent of just one nugget. Servings of baked beans varied across brands from half to one third of a 420g can.

Unclear labelling risks misleading consumers about the amount of salt in their diet, according to the Local Authorities Co-ordinators of Regulatory Services (Lacors), which coordinated the study.

"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," said Geoffrey Theobald, chairman of Lacors.

Environmental health officers at 60 local councils across the UK collected 831 samples of processed foods from supermarkets and other stores.

Tests revealed that the salt content of the foods had fallen by 10.9% since May 2005, though fewer than half the foods met the Food Standards Agency's salt reduction targets for 2010.

The Food Standards Agency has introduced a "traffic light" system of food labelling that lists ingredients as red, amber or green, to indicate whether levels are unhealthy or not. While several retailers have adopted the scheme, others, including Tesco and large manufacturers such as Nestlé and Kelloggs, have championed an alternative system expressing ingredients as a percentage of a guideline daily amount.

Targets set by the FSA aim to see daily recommended levels of salt intake fall from today's level of around 9g to 6g by 2010. According to the pressure group Consensus Action on Salt and Health, a 3g drop in salt intake would lead to a 22% reduction in strokes and a 16% fall in coronary heart disease, preventing 35,000 deaths in Britain every year.

Amelia Lake at Newcastle University's Human Nutrition Research Centre, said: "Manufacturers and people who sell food have a responsibility to consumers, at the very least to be clear about the contents of their products."