Sandwiches supplied to schools and hospitals may have been contaminated with listeria, it emerged over the weekend.
The sandwiches were supplied across London and the South East by Kent-based Anchor Catering.
Anchor Catering, which produces up to 10,000 sandwiches a day, supplied products within the use-by dates subject to the listeria scare - 21 February to 14 March - to schools, hospitals and local authorities across Kent, Sussex, Essex, Middlesex and Greater London before the problem was detected.
Dr Andrew Wadge, the FSA chief scientist, said: "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."
By Daniel Thomas