Three hospital patients have died after eating pre-packaged sandwiches, according to Public Health England (PHE).
A total of six seriously ill patients were struck by listeriosis in the care of Manchester University NHS Foundation Trust and Liverpool's Aintree University Hospital NHS Foundation Trust.
Sandwiches and salads linked to the cases have been withdrawn and supplier The Good Food Chain has voluntarily ceased production while the investigation continues.
Listeria infection in healthy people is usually either unnoticed or causes very mild illness, according to PHE, but it can have more serious consequences for those with pre-existing health conditions and pregnant women.
On average there are 166 annual cases of listeriosis in England and Wales, with 46 deaths.
PHE says the best way to prevent listeriosis is to practice good food hygiene.
It advises chilled food should be put in a fridge or other suitable equipment as soon as it is delivered.
Food that is required to be chilled is not permitted to be left out of temperature control for long periods of time.
Chief operating officer at the FSA Colin Sullivan told the Mirror: "Our sympathies are with the families of those patients who have tragically passed away.
"We have taken action along with local authorities to minimise the risk based on the evidence so far. The FSA will continue to investigate how the outbreak occurred and if further steps are required to protect vulnerable groups."
