Two more patients have died after eating pre-packaged sandwiches and salads linked to a listeria outbreak, taking the total number of deaths to five, Public Health England has said.

The source of the infection is understood to relate to products supplied by The Good Food Chain and the affected sandwiches and salads have since been withdrawn from hospitals.

PHE said evidence suggests all the deceased ate the products before the withdrawal took place on May 25.

The investigations into the outbreak are ongoing, PHE said, and are being conducted in partnership with the Food Standards Agency (FSA).

The first three victims who died were at Manchester University NHS Foundation Trust and Aintree University Hospital NHS Foundation Trust in Liverpool.

It is not yet known where the latest two victims were receiving treatment.

The first case showed symptoms on April 25 and sandwiches and salads were withdrawn on May 25, as soon as a link with the cases was suspected.

It is understood that some of the products were sold at hospitals while others were given to patients.

The Good Food Chain, which supplied 43 NHS trusts across the UK, has voluntarily ceased production.

The business was supplied with meat produced by North Country Cooked Meats, which has since tested positive for the outbreak strain of listeria and also stopped production.

Listeria infection is rare and usually causes a mild illness in healthy people.

However, it can have more serious consequences among those with pre-existing medical conditions, pregnant women and those with a weak immune system.

The Good Food Chain said in a statement: “Our thoughts and deepest sympathies are with the families of those who have died and anyone else who has been affected by this outbreak.

“The underlying cause of it remains unclear.

“We understand that the investigations being carried out are complicated and will involve a number of stakeholders, but we hope the authorities will pursue their inquiries with all urgency so the wider industry can learn any lessons as soon as possible, and so those affected do not have to wait too long for answers.

“For our part, we are co-operating fully and transparently with the FSA and other authorities, and will continue to do so.”

North Country Cooked Meats said previously: “At this stage the company is currently co-operating fully with the environmental health and the FSA in their investigations, and a further statement will be issued in due course.”