Three people have died in hospital in England and three more are seriously ill after an outbreak of listeria linked to pre-packaged sandwiches, Public Health England (PHE) said on Friday.

The deaths happened at Manchester University NHS Foundation Trust and Aintree University Hospital NHS Foundation Trust hospitals, it added in a statement on its website.

The Good Food Chain company, based in central England, has stopped production while an investigation takes place, the state health agency said.

Listeria infection in healthy people is usually either unnoticed or may cause very mild illness.

However, it can have more serious consequences for some people, particularly those with pre-existing health conditions and pregnant women.

PHE said the health risk to the public remains low and individuals should only seek medical attention if they develop symptoms.

"Our thoughts are with the families of those patients who have died. We, along with the 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," said Nick Phin, Deputy Director at the National Infection Service at PHE.

"Our sympathies are with the families of those patients who have tragically passed away," said Colin Sullivan, Chief Operating Officer at the UK's Food Standards Agency.

"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."