Five patients, three of whom have since died, have been diagnosed with the illness, which is caused by the common bug listeria.

The deceased had been ill with other conditions, the trust said.

Meanwhile, the Eastern Health and Social Services Board – one of the agencies helping with the investigation – stressed that any risk to the wider community is small.

And the board said all relatives of patients who were directly affected have been contacted.

All possible causes of the outbreak, including external sources, were being probed last night.

Listeriosis cannot be spread easily from one person to another. It is caused by a bug sometimes found in foods such as pate, soft cheeses, sandwiches and cooked meats.

All such foods have been removed from the menus in the Royal Victoria and other hospitals managed by the trust.

Relatives and patients were also being urged against taking food into wards.

In normally fit and healthy people listeriosis can lead to short-term and self-limiting illness with symptoms such as mild fever and diarrhoea.

However, it can be more serious for people who are already sick from other causes and among pregnant women. Cases of serious illness caused by listeria remain very rare.

Trust medical director, Dr Tony Stevens, said it was “very uncommon” to see five cases.

He revealed that investigations were initiated last week when three or four cases came to light. In a fifth case the patient died on Tuesday.

Dr Stevens emphasised that listeriosis is not a “superbug”.

He explained: “People are infected by this probably quite often and only suffer minor inconvenience. The problem is when very, very sick patients are affected.”

He added that the three patients who died had been very seriously ill and that listeriosis may have had some impact on – or contributed to – their deaths.

Dr Stevens said the trust did not believe the bug had contaminated food in the hospital’s canteens.

“Our food systems are very good and are regularly inspected,” he added.

Last night, fact-sheets were being provided for relatives and patients in wards at the Royal Victoria and a trust information line was launched.

It will be open between 9am and 5pm daily until further notice and can be accessed by calling 0800 9801100.

Additional information on listeria is available from the Health Protection Agency website www.hpa.org.uk (topics AZ).