It is understood the patient - who had recently returned from Portugal was treated with antivirals earlier this week at the Derry hospital.

Geraldine Hillick, the Western Trust's assistant director of emergency care, said: Tests on a patient who attended Altnagelvin Area Hospital earlier this week confirmed that the patient had swine flu.

"Symptoms experienced by the patient were mild and swine flu was diagnosed after admission to hospital for other investigations. The patient received appropriate medical care, was given antivirals and discharged after being in hospital for a short period of time.

"Patients, staff and others who were in close contact with the patient have been identified and received advice and antivirals, as appropriate. The patient had travelled in Europe recently."

Mrs Hillick said: "The Trust advises the public that anyone with symptoms of flu should stay at home and telephone their GP for medical advice.

"The Western Trust with other health and social care organisations has well-developed plans to deal with a flu pandemic. I would like to reassure the public that the Trust has robust arrangements in place.

"An incident group is fully operational and meets regularly to manage the situation as it develops. The Western Trust is working with the Public Health Agency and the DHSSPS to ensure that every precaution is taken and that the Trust is prepared should the situation escalate further."

The Northern Ireland swine flu helpline number is 0800 0514 142. This is a freephone helpline for those who want more information about swine flu.

Further information about swine flu can be found at www.dhsspsni.gov.uk.