SWINE flu is likely to have infected in excess of 45,000 people in Britain - more than 10 times the official figure, experts have warned.

Data released by the Health Protection Agency on Friday revealed that 4,250 people had contracted the virus.

However, John Oxford, professor of virology at Queen Mary, University of London, said the cases confirmed by the agency were likely to be just the “tip of the iceberg”.

“We know it is circulating widely in the community now,” he said. “The cases that have been tested and confirmed should be seen as a sample of those infected rather than as the total number of cases.”

Experts believe the true figure could be 10-20 times higher than that confirmed by the agency. However, there is relief that most of the people infected have had only a relatively mild illness.

Those who contract the virus at this stage are more likely to enjoy some protection if it later mutates and becomes more dangerous.

In America, more than 1m people are thought to have been infected, although official figures confirm only about 27,700 cases.

The swine-flu virus mainly affects people under the age of 50, but those with existing health problems - such as asthma, heart disease, diabetes and obesity - tend to be the worst affected.

The elderly are proving resilient to the virus, possiblybecause they have immunity left from previous epidemics. Last week it was announced in the UK that a policy of ring-fencing the virus has been abandoned in the worst-hit areas of the country. Instead, resources will be targeted at those who are the most at risk from the disease.

Sir Liam Donaldson, the chief medical officer, warned of a likely surge of cases in the autumn. Children are often the most effective carriers and the new school term in September is likely to trigger an increase in the number of cases.

The west Midlands has been the worst-affected region of the country, with 1,516 cases. London has the second-highest number, with 985 confirmed cases, while there have been 850 cases in Scotland.

In Mexico, where the outbreak started in April, there are now 8,279 reported casesand there have been 116 deaths.

The World Health Organisation declared a global pandemic earlier this month. According to its figures, there have been nearly 60,000 cases reported worldwide and 263 deaths.