Up to 1.2 million people could end up in hospital 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 from last year 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".
Top medics have downplayed the predictions, saying factors like Britain's high state of readiness and high immunity levels will lessen the impact.
The bleak forecasts come in "Pandemic influenza: Surge capacity and prioritisation in health services", which was prepared by the Department of Health (DH) 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 one million during a 'normal' season up to 30 million."
There are also grim implications for the demand of the nation's estimated 3,450 adult intensive care beds and 320 children's.
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