Once the virus has mutated into a form easily transmitted between humans, drug companies will set to work developing a vaccine.

The government says it will buy 120 million doses of it - enough to treat the whole UK population.

But a vaccine will only be able to be formulated once the pandemic has begun and will take up to six months to prepare.

Drug manufacturers have been invited to tender for the contract in advance.

A stockpile of 14.6 million courses of the antiviral drug Tamiflu is also being built up. This would not be a cure or offer immunity from the disease but could alleviate symptoms and slow its progress.

It would be enough to treat a quarter of the UK population.

The government has also purchased a limited supply of vaccines against one Asian bird flu strain - A/H5N1. It is not known how effective this would be against a mutated strain being transmitted between humans but it might offer some protection.

And leaflets have been sent to health professionals to advise them about what to expect in case of a flu pandemic.

Health staff are being trained in how to deal with a pandemic and NHS bodies are formulating contingency plans which could be brought into play.

In the time it takes to develop and procure the new vaccine, the following measures would be used to try to buy time and stop the virus spreading:

• Travel between the UK and infected countries restricted and health screening begun at ports

• Infected people asked to go into voluntary isolation and those who had been in contact with them into voluntary quarantine

• Public gatherings such as football matches and concerts restricted, along with movement of people within local areas

• Schools closed

• Infected people, those who had been in contact with them and possibly the general public told to wear face masks.

In addition, government television, radio and newspaper advertisements would give simple advice on hygiene and how to avoid infection.

Once a vaccine was available, it would be administered according to the following priorities:

1. Health workers occupationally most at risk

2. Key service workers

3. Group(s) most vulnerable to illness (this will depend on the strain and will not necessarily include the elderly)

4. Enclosed communities, such as residential care homes

5. People most likely to transmit the virus, such as children

6. General population.

The World Health Organization predicts that one in four of the UK population could be infected in a bird flu pandemic, resulting in a possible 50,000 deaths.

Government plans are based on a quarter of the population being infected at some point during a pandemic. This could rise to 50% in a worst case scenario.

The virus may hit in several waves coming either weeks or months apart.

All age groups will be at risk, though children and otherwise fit adults may be most vulnerable.

Older people might turn out to have some residual immunity due to having been exposed to a similar flu strain earlier in their lives.

The illness is likely to hit sufferers harder than "normal" influenza, with more people developing severe prostration, viral pneumonia and complications or dying.

The likely fatality rate is not yet known, but the government is using a possible overall figure of 48,400 deaths for planning purposes.

The total number of deaths in the UK per week is expected to rise to at least double the normal level of 12,000 at the peak of a pandemic.

At least a third of flu deaths could be in the under-65s, compared with the normal rate of less than 5%.

The government warns the "intense, sustained and nationwide" impact of a pandemic could quickly overwhelm health and social services.

Demand on health services will increase as more people become sick and at the same time the workforce itself is likely to be depleted by sickness, caring responsibilities and deaths.

GPs are likely to see as many as 14 million more patients during a pandemic than normal.

Hospitals will see a rise of at least 50% in admissions for acute respiratory conditions, with at least 20,000 new patients per week needing hospitalization at the peak.

Transport systems, supply networks and utilities are likely to be disrupted by staff sickness and movement restrictions.

This, plus staff sickness, will have a knock-on effect for all workplaces in the UK.

This includes all the emergency services, the military and essential services such as fuel supply, food production and distribution and transport, as well as prisons, education and businesses.

Employers are advised to base their contingency plans on a total of 25% of workers having to take between five and eight days off over a period of three to four months due to sickness and caring responsibilities.

But the virus may spread faster in workplaces, hitting them harder over a shorter period of time.

In the worst case scenario, staff absenteeism is expected to peak at a maximum of 15% at any one time.

However, some employers may be hit harder than others.

The virus is also expected to spread rapidly in schools, where infection rates of 90% are envisaged.

Other institutions such as prisons and care homes will be similarly hit.

Closing schools would have serious implications for the wider society, as parents were forced to stay home from work to care for children.

In addition to maintaining the continuity of their work, businesses are advised to consider how they would protect the security of their premises if they were badly affected by sickness.

They are advised to make plans to run their operations with minimum staffing, and consider redeploying staff to areas they might not normally work in or taking on additional staff or volunteers where necessary.

Employers should also consider providing accommodation, such as bunks in temporary buildings, where staff can rest in between shifts if transport disruption or movement restrictions mean they cannot get home.

According to a study by business risk consultancy Maplecroft, the UK is at "extreme risk" from the spread of a flu pandemic, along with much of Western Europe, the US and Canada.

The firm ranks the UK at number 25 on its list of countries most vulnerable to the overall effects of a pandemic - the highest ranking Western state.

It is ranked ninth in the world on a list of countries where a human version of bird flu is most likely to emerge.

On a list of countries to and within which the virus is most likely to spread, the UK comes top.

Maplecroft says this is because of the openness of the UK economy, its high population density and high numbers of tourists, immigrants and visitors from abroad.

However, on a list of countries which would struggle most to contain the virus, the UK is ranked 148th, indicating a low risk.