The virus has not been contained in Asia

The vaccines will be used to protect key medical and emergency workers across Britain against a possible global pandemic.

The Department of Health has invited manufacturers to tender for a contract.

The World Health Organization says a pandemic of bird flu is inevitable and could cause 50,000 deaths in the UK.

It makes sense to ensure that we in the UK are prepared as we can be Patricia Hewitt Jakarta confirms cases

It has warned that efforts to contain the spread of H5N1 in Asia have not been successful.

Latest WHO figures show there have been 54 human deaths so far - in Vietnam, Cambodia and Thailand.

New fatalies

Indonesian authorities confirmed the first three human fatalities from the bird flu in the country on Wednesday.

There is particular concern that these cases may have resulted from human-to-human transmission.

UK experts will be able to carry out further clinical studies on the H5N1 vaccine to learn more about how it works against the virus and how effective it could be.

Health Secretary, Patricia Hewitt said: "I have decided it would be prudent to purchase a limited quantity of H5N1 vaccine which could be used to help protect those that need it most, such as NHS workers.

"It makes sense to ensure that we in the UK are prepared as we can be.

"This, alongside the purchase of 14.6 million doses of antivirals and the other public health measures we have in place, will help ensure that the UK continues to be at the forefront of international preparedness for a possible flu pandemic."

Professor Ian Jones, a virology expert at the University of Reading, said: "It's good news and builds upon the antiviral drug stockpile announced earlier in the year.

"The only concern, given that the virus constantly changes, is that manufacture can be completed before the virus drifts too far away from the current threatening strain."

Mutation fears

At present H5N1 poses only a limited threat to humans.

On occasions it has jumped from animals into humans - but cannot be spread easily from person to person.

However, experts fear that the strain will eventually mutate to acquire this ability - and warn that when it does thousands of lives will be at risk across the world.

Dr Dick Thompson, a WHO expert, said: "If it acquires the ability to move easily from person to person it could ignite a pandemic.

"It hasn't been contained, it seems to be spreading more widely. Recently we had reports of the virus turning up in migratory birds in western China."

Dr Thompson said H5N1 was very unstable, and its genes were always changing - that alone might be enough to trigger the creation of a more deadly form.

Alternatively, if the virus infected an animal or human also carrying a standard form of flu the two viruses could swap genes in a process called reassortment.

Dr Sandra Bell, director of homeland security for the Royal United Services Institute for Defence and Security Studies, said the UK was closely monitoring the situation, working on diagnostic techniques and putting in place contingency plans to ensure vital services would be maintained.

"We don't know when it is going to come, or what it is going to be like, but we have put a lot of planning and preparation in place so that we can move very, very quickly in order to minimise the impact."

Other countries, including France, Canada, Australia and the US have also announced they will be making or have made arrangements to purchase limited quantities of H5N1 vaccine.