The drugs -- enough to treat about one quarter of the population -- will not prevent an influenza pandemic but could mitigate its effects.

"We need to be prepared for a flu pandemic," Reid told reporters. "We don't know when it will happen ... but we know it is likely it will happen and certainly the risk of it happening and the consequences are high enough to ensure we make preparations."

Health experts fear that the H5N1 bird flu virus that has killed 47 people in Asia could mutate into a strain easily transmitted in humans that could trigger a pandemic similar to the Spanish Flu in 1918 that killed between 20 million to 40 million people worldwide.

So far, though, there is no evidence of sustained person-to-person transmission of the H5N1 virus. Medical experts suggest that around one in four people in Britain could be affected by a flu pandemic. Without counter-measures, they estimate it could kill 50,000 or more people. Seasonal flu kills about 12,000 people each year.

VACCINE WILL TAKE MONTHS TO DEVELOP

An effective vaccine is the only measure that will defeat a flu pandemic but it could take up to six months after the strain is identified to develop a vaccine against it. Tamiflu is the drug of choice to reduce the impact of the disease.

"We can't prevent the pandemic from coming but we have a good plan to mitigate its effects," said Sir Liam Donaldson, the chief medical officer.

Influenza pandemics are caused when a new flu virus emerges, so ordinary flu vaccines will not be effective against it.

Flu pandemics, which are thought to have originated in China, have also occurred in 1957-1958, when an estimated 1 million people died worldwide, and in 1968-1969 in which 1 million-4 million died.