London (CNN) Three people have died and three are seriously ill following an outbreak of listeria infection in hospitals in England, Public Health England has said.

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.

The Good Food Chain, which distributes sandwhiches and salads linked to the outbreaks, has halted production and withdrawn potentially contaminated produce from hospitals.

"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.

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