A 19-year-old man has become the fourth person in Britain to die after contracting swine flu and the first fatal case in London, officials announced yesterday.

The unnamed man, from South London, had serious underlying health problems and it is not known if the flu infection contributed directly to his death.

He died on Wednesday but was confirmed to have the H1N1 virus only afterwards.

Simon Tanner, regional director of public health for London, said: “It is with sadness that we have to announce the death of a patient in London who has subsequently tested positive for H1N1 swine flu.

“No further details will be released to protect patient confidentiality. Our thoughts are with [his family] as they come to terms with their loss.”

All the people who have so far died in Britain and have tested positive for swine flu were already suffering from underlying health problems, which may have made them vulnerable to infection or complications.

There have been at least 7,500 confirmed cases of swine flu in Britain so far, and Dr Tanner emphasised that in most cases the infection was a mild illness from which people could recover in a few days without needing to see their GP or go to hospital.

“However, some people may be more susceptible to the virus,” he said. “If you have flu-like symptoms it is important that you talk to your doctor if you are pregnant, very young, over 65 or have long-term conditions, such as asthma or diabetes.”

Jacqueline Fleming, 38, of Glasgow, was the first person in the UK to die after catching swine flu. Ms Fleming died last month, two weeks after her son, Jack, was born 11 weeks prematurely. Jack died later but had not been infected. The second victim was a 73-year-old man from the Inverclyde area of Scotland.

A week ago today, Sameerah Ahmad, 6, became the youngest UK victim to date. Sameerah, who was born with a rare, life-threatening disease, died at Birmingham Children’s Hospital. It is unclear whether swine flu contributed to her death.

Sir Liam Donaldson, the Chief Medical Officer for England, said on Thursday that more research was needed to understand the nature of the virus.

He said that the deaths of young and previously healthy people had been reported in other countries after they contracted swine flu, and that

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