Three Britons have died of food poisoning after eating pre-packaged sandwiches, UK health authorities said Friday, stressing that the risk to the wider public remained “low”.

The patients died of listeriosis, a rare bacterial infection that is especially dangerous for people with weak immune systems or pregnant women.

The listeria bacteria primarily develops in dairy products and chilled ready-to-eat food, according to Britain’s National Health Service (NHS).

Public Health England said “a number of local authorities are currently investigating the source of listeria infections linked to pre-packed sandwiches”.

“The cases of listeria infection were in six seriously ill hospital patients in England. Three of these patients have sadly died,” the health service said.

It said the sandwiches’ supplier, The Good Food Chain, had voluntarily ceased production while an investigation is underway.

Nick Phin, the deputy head of the National Infection Service, said no other cases have been identified “and any risk to the public is low”.

Health authorities did not specify where the cases developed or how widely the sandwiches are sold.