Dr Rachel Thompson, science programme manager for the World Cancer Research Fund (WCRF), said that a single serving of some leading brand soups contains half the recommended daily maximum intake of salt.
She said that soups are one of the "worst culprits" for hidden salt, and recommended people make their own low-salt versions.
As well as increasing blood pressure and the risk of stroke and heart disease, scientists also believe that salt is a cause of stomach cancer.
Dr Thompson advised that salt levels can be kept down not only by watching the salt content in soups, but also by reducing intake of salt-preserved foods, such as hams and sausages, as well as pizza, some ready meals and breakfast cereals.
The Government's recommended daily maximum intake of salt for adults is no more than 6g a day, although people currently consume an average of 8.6g daily.
Dr Thompson said: "Soups are one of the worst culprits for hidden salt. Some brands of soup have as much as half the recommended daily intake per serving."
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