Researchers found that the New Covent Garden Food Company’s Our Best Chicken Soup contained 6.25 grams of salt in a 250-gram serving, the same concentration as would be found in seawater.

The Food Standards Agency recommends that adults should not consume more than six grams of salt a day. High salt consumption is linked to high blood pressure and is blamed for increased risk of heart disease and stroke.

The research, by Consensus Action on Salt and Health (Cash), found high levels of salt in a large number of popular brands of soup. Among varieties by Campbell’s was a condensed oxtail soup with 3.1 grams of salt per 250 grams, more than half the daily allowance. Five other Campbell’s soups had 2.5 grams of salt per 250 grams.

Soups from Marks & Spencer criticised in the survey were tomato and lentil and country vegetable, both of which had 3.1 grams of salt per 250 grams.

Melanie Johnson, the Public Health Minister, has given processed food producers until Friday to prove that they are taking steps to reduce salt content, and has said that tough government action will follow if she is not satisfied.

Manufacturers have been urged to display salt content on labels rather than quoting the content as sodium. Shoppers have to multiply the amount of sodium by 2.5 to calculate the equivalent as salt.

Professor Graham MacGregor, the chairman of Cash, said: “You can see from the survey that one relatively small serving of soup has a great deal of salt. And this is in food that is considered a snack or perhaps a starter, so it is a major source of hidden salt that people don’t realise.”

The New Covent Garden Food Company has agreed to reduce the amount of salt in its cartons of Our Best Chicken Soup, each of which at present contains 15 grams.

Times Recommends

• Third of Muslim students back killings
• Curfew tames feral yobs of Cornwall
• £3,000-a-month drug free abroad