Millions of people in Britain could be infected with swine flu and thousands of schools could be closed as the first flu pandemic for 40 years was officially declared by the World Health Organisation.
Describing the virus as "unstoppable", Dr Margaret Chan, director-general of the WHO, said "the world is now at the start of the 2009 influenza pandemic", adding further cases and deaths linked to swine flu are inevitable.
The Department of Health has told NHS managers to expect a surge in cases of H1N1 in the autumn when children go back to school after the holidays and again over Christmas.
Managers were warned that one in four staff could be ill with the virus and others may have problems getting to work because of sickness in their family.
Professor Neil Ferguson, of the MRC Centre for Outbreak Analysis and Modelling at Imperial College London, said widespread illness could lead to mass school closures. At least 20 schools in England and Scotland have been closed for up to a week after pupils tested positive for the H1N1 strain.
Estimates suggest a widespread school closure policy could prevent one in seven cases and one in five among children. But the advantage would have to be weighed against the economic and social costs of such a move.
Sir Liam Donaldson, Britain's chief medical officer, said several million people in Britain could be infected, putting the NHS under pressure.
But he said if there were more widespread cases in Britain then the Government should review the policy of shutting schools with cases, as closure would be more disruptive than just treating affected pupils.
The pandemic definition describes the geographic spread of the disease rather than its severity and Dr Chan said the pandemic is expected to be moderately severe as things stand.
She told a press conference in Geneva the majority of cases have been mild with most people making a full recovery without the need for medication.
However the H1N1 swine flu virus is striking young and middle aged people more frequently and in around two per cent of cases the disease had rapidly progressed to life-threatening pneumonia, Dr Chan said.
There have been almost 30,000 confirmed cases and 144 deaths worldwide, with cases in one third of countries. However experts believe the disease may have been spreading in Mexico, the epicentre of the outbreak, for several months before it was identified.
Another 25 cases of swine flu were confirmed yesterday in England alongside 26 in Scotland, taking the UK total to 848. Malvern College, the Worcestershire boarding school, which charges up to £27,600 per year, said it would partially close for 10 days after six cases were confirmed.
One case involved a member of staff, the school said, while a further 10 cases were under investigation.
Andy Burnham, the new Health Secretary, said the declaration of a pandemic did not change the situation for Britain. Confirmed cases and their close contacts have received the anti-viral drug Tamiflu and orders have been placed for 90 million doses of H1N1 vaccine which are due to be delivered by December.
Dr Maureen Baker, Royal College of General Practitioners, said: "This does not mean that patients are in any more danger, or that the virus has become any more harmful. The majority of H1N1 cases will get a nasty dose of flu and recover within a week or two."
Meanwhile the Health Protection Agency has admitted it failed to identify an outbreak of swine flu at Welford Primary School in Birmingham in the early stages as doctors thought it was norovirus or a stomach bug.
In some cases the swine flu virus has caused vomiting and diarrhoea. A total of 96 cases were later confirmed at the school, in the Handsworth area of the city, which was shut and re-opened after the half-term break.