Up to 1.2 million people could be hospitalised and 750,000 killed if a flu pandemic sweeps the nation, according to draft Government guidance about an outbreak.

As hospitals are "rapidly" overwhelmed by patients, doctors may have to begin operating a lottery system for intensive care, the blueprint adds.

It also warns that a worst-case pandemic scenario of "catastrophic severity" could result in the "complete or partial collapse of some or all hospital infrastructures".

The bleak forecasts come in "Pandemic influenza: Surge capacity and prioritisation in health services", which was prepared by the Department of Health last September.

It says up to half the UK population - or 30 million people - could get influenza if the bug outbreak turns into a pandemic.

In the worst case, there would be 2,000 hospital admissions per 100,000 people - or 1.2 million people. There would also be 1,250 fatalities per 100,000 - or 750,000 people.

The report says: "Over the entire period of a pandemic, up to 50% of the population may show clinical symptoms of influenza.

"This could result in the total healthcare contacts for influenza-like illness increasing from around 1 million during a 'normal' season up to 30 million."

There are also implications for the demand of the nation's estimated 3,450 adult intensive care beds and 320 children's.

"It is likely that, in a pandemic, the demand for critical care will be high and the current 3,450 adult critical care beds and 320 paediatric critical care beds in England could be rapidly overwhelmed," the report says.

It adds: "In the face of high demand there may be patients between whom the clinicians cannot differentiate on the basis of benefit.

"At this stage allocation of (intensive care) treatment may require to be by a random selection (lottery) process taking into account the principles of the ethical framework."

Past data suggests a pandemic would last around 15 weeks.

Top medics have downplayed the predictions, saying factors like Britain's high state of readiness and high immunity levels will lessen the impact.