Health Protection Authority officials believe a hospital patient caught listeria after eating one of a batch of contaminated sandwiches later recalled across the southwest.

The sandwiches had been distributed by Anchor Catering across Surrey and southwest London.

On Monday, the HPA said the patient, who recovered from listeria but died of an existing illness, had the same strain as Anchor.

Kent-based Anchor Catering said it had not been formally notified about the development and could not comment.

In a statement, the HPA said: "The strain is relatively uncommon and it is very likely that the cases are linked.

"An individual with a weakened immune system, who reported eating a likely-to-be-Anchor sandwich in a hospital supplied by Anchor, tested positive for the same strain of listeria as the one found at Anchor Catering.

"We understand from the hospital that the individual recovered from listeria, however they subsequently died from separate complications caused by the illness that they were receiving hospital treatment for."

The HPA has not said which hospital was involved.

The Food Standards Agency recalled the sandwiches on March 16.

At the time, no cases of listeria had been reported, but the FSA said symptoms could take up to 90 days to appear.

The sandwiches, with use-by dates from 21 February to 14 March and labeled either Anchor or Pomegranate, had also been supplied to schools and businesses across southwest London and Surrey as well as Kent, Sussex and Essex.

The FSA warned large numbers may have been eaten before the problem came to light during routine sample test by Ashford Borough Council.

After the alert, Anchor voluntarily suspended production at its sandwich factory, and recalled the products.

The FSA said it was investigating but said most illness caused by listeria could be treated with antibiotics.

At greatest risk are pregnant women, the elderly, young babies and people with weakened immune systems with initial symptoms including fever, a flu-like illness or diarrhoea.