Listeria has an incubation period of up to 70 days, so further cases could emergeGETTY IMAGES

The food safety watchdog was warned about the risks of sandwiches supplied to hospitals two years before a fatal outbreak of listeria last month.

Three patients died in the northwest of England and three more became seriously unwell. Scientists linked the outbreak to chicken sandwiches from The Good Food Chain in Stone, Staffordshire. It voluntarily withdrew the sandwiches on May 25 but the link was not made public until last Friday.

A report for the Food Standards Agency watchdog in 2017 warned about the risks from sandwiches being supplied to NHS hospitals.

Fiona Sinclair, a food safety consultant, said that studies had found that about 3 per cent of sandwiches served in hospitals and care homes were contaminated with listeria. The majority of listeria outbreaks…