Sir Liam Donaldson, the chief medical officer, said the Government will buy 120 million doses of the vaccine, which will be enough to immunise 60 million people.

However, Sir Liam admitted that a vaccine cannot be made until scientists know which virus to base it on, and that is only possible once the pandemic strain has emerged.

"A vaccine to protect against pandemic flu cannot be made until the new virus is known. However, there are steps we can take to reduce the time before manufacturing starts," Sir Liam said.

One measure the Government has agreed on is to set up "sleeping contracts" with drug companies to build up production facilities ready to make a vaccine as soon as the strain of virus is identified, Sir Liam said.

"We are inviting manufacturers to tender for a contract to supply future requirements for around 120 million doses of a pandemic vaccine. We will use this vaccine to immunise the UK population and reduce the impact of a pandemic on society," he said.

Sleeping contracts will allow companies to spend money on boosting their vaccine production facilities without the risk of financial losses.

The contracts are part of the Government's contingency plan for limiting the spread and impact of pandemic flu, which also include the purchase of 14.6 million doses of antiviral drugs.

"One of the most effective countermeasures we can take against a flu pandemic is to make sure we develop and manufacture a vaccine as quickly as possible," Sir Liam said.

The H5N1 strain of bird flu virus has the potential to develop into an infectious human virus but so far that has not happened and until it does no effective vaccine can be made, he said.

"We can't have tunnel vision and assume it will happen from the current bird flu strain, although it's the most likely," Sir Liam said.

The Department of Health estimates that it will take between four and six months to develop an effective vaccine once a pandemic strain of flu is isolated. However, a pandemic is likely to spread faster than a vaccine can be made, Sir Liam said. But a vaccine would still be useful because flu pandemics often occur in successive waves, and if a vaccine is not ready for the first wave, it could limit the impact of a second or third wave, he said.

"Past experience suggests that a second, and possibly further waves of illness caused by the virus are likely three to nine months after the first wave. The second wave may be as, or more intense than, the first," Sir Liam said.

As part of the contingency plans, Government officials are considering a wide range of options to limit the spread of a pandemic flu virus within the population. These measures could include closing schools, banning large gatherings of people at public events or issuing face masks.

The Department of Health will issue information to GPs later this week explaining what they need to do in the event of pandemic flu reaching Britain.

Sir Liam said that although it is not possible to prevent a flu pandemic once it has begun, it is feasible to reduce its impact.

"We need to be doing things now in the pre-pandemic phase," he said.

The countermeasures

* Enough vaccine for 60 million people, each given two shots from a stockpile of 120 million doses

* A stockpile of 14 million doses of antiviral drugs to be given to people in greatest need

* Facemasks to be issued to all medical personnel and possibly members of the public

* Government may ban mass gatherings of people

* Schools may be closed

* People urged to improve their personal hygiene, wash their hands and stay at home if they fall ill