A teenager with serious pre-existing illnesses has become the fourth Briton to die from swine flu, the National Health Service said on Friday.

Simon Tanner, director of public health for London, said the 19-year-old was the first person to die from the infection in the capital since the outbreak spread from Mexico to Britain in April.

"(He) had serious underlying health problems," Tanner said in a statement. "No further details will be released to protect patient confidentiality."

The World Health Organization declared on June 11 that the outbreak of the virus was a pandemic. More than 77,000 people have been infected worldwide, including about 7,500 confirmed cases in Britain.

Most people who have caught the infection have suffered mild symptoms, but in a small minority it has proven more severe.

Health Minister Andy Burnham said on Thursday Britain is projecting more than 100,000 new cases a day of the H1N1 flu by the end of August and will change the way it deals with the virus.

The number of confirmed cases of the virus is doubling every week, putting pressure on health services, he added. Health agencies will no longer try to contain the virus.

"We have always known it would be impossible to contain the virus indefinitely and that at some point we would have to move away from containment to treating the increasing numbers falling ill," Burnham told parliament.