Soup as salty as seawater has been found on sale, highlighting concerns about the impact of processed food.

Researchers discovered a chicken soup from the New Covent Garden Food Company with 6.25g (more than a teaspoon) of salt per 250g bowl, equivalent to the level found in seawater. The Food Standards Agency recommends adults consume no more than 6g a day.

Diets rich in salt are linked to high blood pressure, which increases the risk of heart disease and stroke.

The research by Consensus Action on Salt and Health (Cash) found worrying levels of salt in a large number of popular brands of soup.

Among varieties sold by Campbell's was a condensed Oxtail soup with 3.1g of salt per 250g bowl - just over half of the recommended daily total. Five other Campbell's soups - Scotch Broth, Vegetable, Asparagus, Cream of Tomato and Red Pepper, and Italian Tomato and Basil - had 2.5g of salt per bowl.

The same amount was also found in three condensed Campbell's soups promoted as 99 per cent fat free - Chicken, Tomato, and Tomato and Red Pepper.

Soups from Marks and Spencer were also criticised by Cash. Two varieties - Tomato and Lentil, and Country Vegetable - had 3.1g of salt per bowl or 5.3g in a whole 425g tin.

The New Covent Garden Food Company has agreed to cut the amount of salt in its cartons of Our Best Chicken Soup, each of which contains 15g. The company's other soups were generally among the least salty in the survey. Melanie Johnson, the Public Health minister, has given processed food producers until 27 February to prove to her that they mean to cut salt or they could face tough new measures.

Manufacturers have been urged to display the salt content on labels rather than sodium. Shoppers must multiply the amount of sodium by 2.5 to find the salt figure.

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 if you add in a couple of slices of bread, that's another gram of salt.

Marks & Spencer and Campbell's said that they were working to reduce the amount of salt in their soups and other foods.