Thousands of schools could be forced to close for weeks on end to stem the spread of swine flu, scientists say.

As the World Health Organisation (WHO) declared the first flu pandemic for 40 years officially under way, the Government’s Chief Medical Officer said that “several million” Britons would fall ill with the H1N1 virus.

Sir Liam Donaldson predicted a “huge surge” of cases when children go back to school in the autumn.

Neil Ferguson, an epidemiologist at Imperial College, London, who advises the Government and the WHO, said that widespread illness could lead to schools closing for several weeks, and extensive disruption to businesses and the wider economy. The UN Health Agency decribed the virus as “unstoppable and “unpredictable” after nearly 30,000 cases were confirmed across several continents. So far, only 822 cases of swine flu have been confirmed in Britain, but Professor Ferguson said that the pandemic could mean that 25-35 per cent of the population would fall ill within three or four months, placing severe strain on the NHS.

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The virus was likely to be two to three times more deadly than seasonal flu, which kills an estimated 12,000 people each year, he said.

Shutting schools has already been used as a means of slowing transmission of the pandemic, with at least 20 in England and Scotland closing for up to a week after pupils tested positive for the H1N1 strain.

It is estimated that a widespread school closure policy could prevent one in seven cases and one in five among children, while reducing infections at the peak of the pandemic by 40 per cent.

A decision will be not be taken until the circumstances of any autumn outbreak are known, however, because of the measure’s high economic and social costs. Working parents would have to make alternative childcare arrangements for several weeks, adding to absenteeism because of illness.

However, Sir Liam also cautioned that the illness could be so prevalent at the peak of a pandemic that even large-scale school closures could not contain its spread.

The last global flu pandemic, in 1968 — “Hong Kong” flu — killed up to four million people worldwide, including 25,000 in Britain.

Although most cases of illness caused by H1N1 have so far been mild — with 144 deaths recorded worldwide and none in the UK — Sir Liam said that past pandemics suggested that there would be a steady increase of cases over the summer in Britain, followed by large peaks in September/October, or at Christmas.

He added that between 400,000 and 800,000 people became ill in an average flu season, “but [at the peak of a pandemic] you would probably be into several million cases”.

At present, the UK has enough antiviral drugs to cover half the population but orders are in place for 50 million doses. Pharmaceutical companies are working on a specific swine flu vaccine, which will not be available for a few months yet.

The WHO’s decision to raise the global alert level from five to six — officially signalling a pandemic — came after a day-long emergency meeting of health officials to discuss the implications of widespread outbreaks of illness in 74 countries, including a sudden surge of cases in Australia.

Andy Burnham, the Health Secretary, said that Britain’s pandemic plan was already in action and had been modified to contain local outbreaks.

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