Food firms accused over labels peppered with lies about salt

But the serving sizes they use are often artificially small. One firm gave what appeared to be a very low salt level for its chicken nuggets.

However, this was based on a serving of just 15g, which equates to a single chicken nugget, rather than the five or six that a child might eat.

Similar problems are occurring across all food products. For example, one breakfast cereal manufacturer may use a small serving size of 30g for its nutrition claims, while another will use a more realistic 40g.

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A shopper trying to compare the health merits of the two products would need a calculator and an extra hour in the supermarket to do the maths.

The problems have been highlighted in a study of 831 samples of processed food bought in supermarkets by officials from 60 local councils across the country.

They found that some bread companies base nutrition claims on a serving size of 20g, which is one slice, while others go for 50g, which is around two and a half.

Serving sizes used for the nutrition label on a 420g can of baked beans ranged from one-third to a half. Tomato sauce servings varied from 10g to 30g.

The research was organised by the Local Authorities Coordinators of Regulatory Services (Lacors) whose chairman, Geoffrey Theobald, 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."

The hidden salt in processed foods is blamed for many of the nation's health problems. High salt content is linked to raised blood pressure, strokes and heart attacks. The average adult consumption is 50 per cent higher than the recommended level, at 9g a day.

The research found salt levels have come down by 11 per cent since May 2005. But fewer than half the foods tested met the Food Standards Agency's salt reduction targets to be met by 2010.

The labelling is further confused by the fact that companies are only legally required to list sodium, rather than salt, on their packets.

This needs to be multiplied by 2.5 to give the salt level. Lacors argues that portion sizes should be standardised for different groups of products.

And the lobby group Consensus Action of Salt and Health, said: "Unrealistic portion sizes, such as one chicken nugget, make the information misleading for consumers."

Julian Hunt, spokesman for the Food & Drink Federation, which represents manufacturers, said: "Far from hoodwinking, we are introducing some of the most open and honest labelling in Europe.

"Not only do we clearly declare salt levels on the back of packs, many are also putting this information on the front of packs."