The move to alert phase 6 is likely to trigger the fast-track production of vaccines to treat the virus.
The WHO called the meeting after a steep rise in the number of cases in Australia. The country has seen a four-fold increase in cases in just one week.
Worldwide, the virus has spread to 74 countries, infecting more than 27,700 people and causing more than 140 deaths.
A total of 797 cases have so far been confirmed in the UK, according to the HPA.
Steve Barnett, chief executive of the NHS Confederation, said: ‘The confirmation of a level 6 global pandemic reinforces the need for the NHS to ensure all the flu plans already in place at local level are as comprehensive as pos-sible and thoroughly tested.
‘We need to avoid compla-cency in dealing with a virus that is an unknown and seems to be spreading quickly.'
Today health service leaders meeting at the NHS Confederation conference in Liverpool have been briefed by the Department of Health on the scale of the challenge facing them and they understand they have a window of opportunity before winter, when the spread of H1N1 is expected to accelerate.'
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