According to the Department of Health, the UK was the first country to develop contingency plans against a flu pandemic.

The World Health Organisation called on all member states to develop plans in the event of a pandemic two years ago and the UK drew up guidelines by March 1997.

Flu pandemics are rare and only happen when a radically new or changed strain of the virus emerges, meaning that no-one has a chance to develop immunity against it.

This century there have been pandemics in 1918, 1957 and 1968.

The Department of Health says there is no reason to expect that another pandemic is imminent, although they are unpredictable.

Localised outbreak

The current flu crisis is nowhere near pandemic levels.

Tim Jones, a spokesman for the NHS Confederation, said the current outbreak was "not unusual" and was localised.

The Hong Kong chicken flu was brought under control

He added that some hospitals which had reported wall-to-wall cases in accident and emergency two days ago were now saying they could see people with even minor injuries within one and a half hours.

"There are just spikes of demand," he said.

In pandemics, up to 25% of the working population are off work at any given time, including a proportion of health staff.

This can create severe problems. Another concern is the fact that pandemics sweep in so quickly that it is difficult to develop an effective vaccine in advance.

These factors lead to a much higher death rate, often in age groups normally least affected by flu.

Phased response

The Department of Health's Influenza Pandemic Plan sets out a phased response to a huge flu outbreak.

Phase zero is the period between pandemics; phase one is the emergence of a new strain of the flu virus; phase two is outbreaks outside the UK; phase three is outbreaks in the UK; phase four is a pandemic and phase five is the post-pandemic phase.

The Public Health Laboratory Service monitors flu outbreaks worldwide and can provide an early warning system for health authorities.

The last most serious outbreak it tracked was last year's Hong Kong chicken flu.

This is believed to have had the potential to develop into a pandemic, but prompt action by the authorities is thought to have prevented any major global outbreak.

Each health authority and trust has its own action plan for dealing with a huge surge in demand for services.

Most are an exaggeration of what hospitals are doing to deal with the current flu outbreak.

More beds and staff

Treating flu outbreaks mainly requires extra beds and extra staff.

Therefore hospitals attempt to make more space for emergency patients by freeing up additional beds.

Most have mothballed wards which they have closed for financial reasons. These can be opened up again if staff can be found to man them.

If a pandemic struck, hospitals would draft in extra nurses and cancel leave

Non-urgent operations can be cancelled to free up more beds.

If all non-urgent operations are suspended as many as half of a hospital's beds can be made available.

Hospitals can also employ people to walk the wards on a regular basis to check whether there are any patients who are fit enough to be discharged, often in liaison with social services.

Most also have a major incident control centre which, according to Tim Jones, functions like "a Battle of Britain control room" and ensures that all available beds are used.

This would be staffed by managers around the clock.

Hospitals may also suspend leave, may ask staff to work extra shifts and divert staff from non acute care as well as using bank and agency staff.

And they may prioritise the kind of people who get hospital beds, only allowing the most frail to be treated in hospital.

In extreme cases, hospitals may rent hotel wings for extra patients or tents.

Liverpool

The Royal Liverpool Hospital, which has come under severe pressure since the end of December because of the flu, says it is already putting some of the emergency steps into action.

More then 6% of staff are off sick and some wards have had to be closed due to power and hot water failures.

Day wards have had to be used for emergency patients and additional wards refurbished.