Marks and Spencer confirmed yesterday that it had withdrawn thousands of sandwiches from stores after traces of salmonella were found at the supplier's factory.

Routine checks at Telfer's of Northampton, which produces 400,000 sandwiches a week for the store chain, revealed traces of bacteria and the factory was closed last Wednesday while health officials investigated. Marks and Spencer said it would remain closed until it and Telfer's were satisfied there was no danger.

A spokeswoman for Telfer's said the factory was undergoing extra cleaning and staff were being given health checks. She added: "Traces of bacteria were found during the weekly quality assessment test and we told Marks and Spencer immediately and they withdrew sandwiches from their stores."

The firm said that normal production would only resume when it was sure it was safe to do so. Final results of tests are expected this weekend.

Martin van Zwanenberg, Marks and Spencer's technical director, said that his company and Telfer's had reacted as quickly as possible once the test results were through.

"The sandwiches involved had obviously been sold but as a precaution we decided together with Telfer's first of all to withdraw all the sandwiches we had on display that Telfer's had made in the previous week and also to close the factory while we investigated the cause of this one result.

"All subsequent results, by the way, have been clear. We and Telfer's reacted on the day the results came out. We acted immediately," he told BBC's One O'Clock News.