Plans to purchase enough vaccine to cover every person in the UK in the event of a flu pandemic have been announced by the Department of Health.
There are fears the bird flu virus will mutate
Chief Medical Officer Sir Liam Donaldson said 120 million doses would be needed - two for everyone.
But a vaccine cannot be developed until the exact strain of the flu virus causing a pandemic is known.
And Sir Liam admitted cases of flu could be seen in the UK before a vaccine was ready.
That is because it takes four-to-six months from the time a pandemic flu strain emerges to develop and manufacture a vaccine.
It was also announced on Wednesday that the ability of EU nations to coordinate their efforts to tackle a flu pandemic is to be tested in a simulation exercise.
An outbreak of flu in birds has been reported in poultry in the Russian province of Tula, to the south of Moscow.
And China has revealed that 2,600 birds have died from the disease in Inner Mongolia.
Romania has also confirmed birds infected in a second outbreak in the country did have the deadly H5N1 strain of the virus.
Sir Liam published updated guidance on Wednesday on how the UK would prepare for a possible pandemic.
The government is inviting manufacturers to tender for a contract to supply the pandemic flu vaccine.
25% of the population - 15 million people - may suffer from flu during a pandemic
Out of a population of 100,000 people, local health care organisations should expect to see at least 1,000 new flu patients a week
This figure could rise to 5,000 a week at the virus's peak
Around 25% of the working population may need to take sick leave of between five and eight working days
Source: Department of Health
Capacity for two doses per person have been ordered because research so far shows two doses are needed to protect people against H5N1 - the type of virus most likely to mutate into a pandemic strain.
Sir Liam said that offering these "sleeping contracts" now should mean manufacturers can develop enough capacity to make a vaccine.
The CMO added that thousands of information packs were being sent to GPs, including information which can be given to patients.
Sir Liam said: "We cannot prevent a flu pandemic, but we can reduce its impact.
"One of the most effective counter-measures we can take against a flu pandemic is to make sure we develop and manufacture a vaccine as quickly as possible.
"We will use this vaccine to immunise the UK population and reduce the impact of a pandemic on society."
But he stressed there was no direct threat to the UK public from current outbreaks of bird flu in other countries.
The crunch will come if a strain of bird flu - probably H5N1 - mutates, and gains the ability to pass easily from human to human.
Sir Liam has predicted that the epicentre of any new strain is likely to be in East Asia, and that a major outbreak is unlikely in the UK this year.
But he has also warned that at least 50,000 lives could be lost in the event of flu taking hold in the UK.
The UK Influenza Pandemic Contingency Plan includes quarantine measures, stockpiling of anti-viral drugs and research into a vaccine as well as arrangements for the emergency services.
Banning public gatherings, such as concerts and football matches, and restricting travel are measures which could be introduced in the event of an outbreak, to stop the virus spreading.
The UK has so far stockpiled 2.5 million courses of anti-viral drugs, and has ordered a total of 14.6.
Sir Liam said that Roche, which manufactures the drug, had originally agreed to deliver the full order by March 2007, but under government pressure, it had agreed to bring the deadline forward to next September.
Andrew Lansley, the Shadow Health Secretary, said better plans should have been put in place before now.
He said: "The UK Government's plan may be better than many other countries, but it has been slow off the mark and delivery of key elements has not even begun."
The European simulation exercise will involve officials in command centres across Europe reacting to imaginary scenarios. No healthcare staff will be mobilised.
Other European countries are stepping up measures to prevent the spread of bird flu, following the discovery of the H5N1 strain in Turkey and Romania.
The European Commission has banned bird and poultry imports from the countries.
H5N1 has killed more than 60 people in South-East Asia since 2003.
On Monday, Greece became the first EU member to confirm a case of bird flu.
It is not yet known if the Greek case is H5N1.