Salmonella alert issued over supermarket basil

Affected fresh packets of the herb were on the shelves at Asda, Sainsbury's and Somerfield. The Food Standards Agency said that eating the products could lead to diarrhoea and sickness.

A warning on the agency's website said that much of the basil was close to, or already past, its "best before" and "display until" dates.

The statement added: "The affected basil is also labelled as wash before use. However, it's possible that people may still have some affected batches at home.

"For example, they may have used the basil to make fresh pesto sauce or used it as an ingredient in other uncooked foods. Also some people may have frozen the herb."

Sainsbury's and Somerfield have told the agency they have withdrawn all their potentially affected basil stocks with a "best before" date up to and including May 28.

Asda has told the agency that the only affected batches sold through its stores are those with a "display until" date up to and including May 18.

"Agency advice is that people who have bought basil from Sainsbury's and Somerfield with a 'best before' date up to and including May 28 should take it back to the store where they bought it," said the authority's website.

"People who bought basil from Asda with a 'display until' date up to and including May 18 should also return it to the store they bought it from."

Sainsbury's took out national newspaper advertisements at the weekend to highlight the product recall. The contamination came to light during a fresh herb survey conducted by the Health Protection Agency and the local council advisory body, Lacors.

The FSA is conducting further urgent investigations to establish the likely source of the problem in the supply chain and to ensure that no further contamination takes place.

A spokesman said the herb could have picked up the bacteria where it was grown in Israel from contact with birds or their droppings.