Sandwiches supplied to schools and hospitals may have been contaminated with listeria, the food watchdog has warned.
Pregnant women, the elderly and people with low immunity are most at risk of catching the infection, according to the Health Protection Agency.
The sandwiches were supplied across London and the South East by a Kent-based catering company.
Listeria contamination came to light in sandwich samples during routine tests carried out by Ashford local authority.
Symptoms of the infection can take up to 90 days to appear. They include fever, a flu-like illness and diarrhoea.
The Food Standards Agency said no cases of the illness caused by the sandwiches had yet been reported.
Kent-based Anchor Catering produces up to 10,000 sandwiches a day. It supplied sandwiches within the use-by dates subject to the listeria scare to schools, hospitals and local authorities across Kent, Sussex, Essex, Middlesex and Greater London before the problem was detected.
The Food Standards Agency (FSA) has launched an investigation into how the contamination occurred.
FSA chief scientist Dr Andrew Wadge said: "Listeria doesn't pose a threat to the vast majority of the population but it can be serious for some vulnerable groups such as pregnant women.
"We are working closely with the company and the local authority to discover how this happened. Although the company took prompt action to notify customers and withdraw the product, large numbers of sandwiches may have been consumed before the problem was detected."
