Multiple health agencies and local authorities are investigating the source of the infection, which is currently being linked to pre-packaged sandwiches available in English hospitals.
All six people affected were seriously ill in the hospital at the time they were infected, Public Health England said in a statement Friday.
The people who died were being treated at two hospitals in the northern English cities of Manchester and Liverpool, the health agency said.
“Our thoughts are with the families of those patients who have died,” Nick Phin, the deputy director at the UK’s National Infection Service at Public Health England, said in the statement.
“We, along with the Food Standards Agency (FSA), colleagues in local authorities and the NHS have worked quickly to determine the likely cause of this outbreak and taken action to reduce the risk to the public’s health.”
He added that there have been “no associated cases identified outside healthcare organizations,” and said the risk to the public was low.
Colin Sullivan, chief operating officer at the FSA, said his agency had taken action alongside local authorities to “minimize 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,” he added.
Public Health England said the sandwiches and salads linked to the cases of illness were removed from hospitals by the supplier, The Good Food Chain. The agency also said the company voluntarily halted production while the contamination is under investigation.
“Following investigations by Public Health England and the Food Standards Agency, The Good Food Chain has confirmed that its production facility in Stone, Staffordshire was cross-contaminated by an ingredient from one of its approved meat suppliers,” the company said in a statement to CNN.
The meat supplier, North Country Cooked Meats, and its distributor, North Country Quality Foods, have also voluntarily ceased production while the investigation is ongoing, Public Health England said.
CNN has contacted North Country Quality Foods for comment.
Symptoms of Listeria infection include fever, muscle aches, and gastrointestinal illness such as diarrhea.
Pregnant women, newborns, older adults and anyone with a weakened immune system are most at risk for Listeriosis infection. The risk for pregnant women is that the illness can cause miscarriage, stillbirth or premature birth.
Public Health England noted that the National Health Service in England and Wales typically sees 166 cases of listeria infection a year. An average of 46 deaths per year due to Listeria were reported between 2010 and 2016.