A flu pandemic in Britain could kill 750,000 people and leave up to 1.2 million in hospital, draft government guidance about the disease has stated.
And an influx of patients on to wards could force doctors to start operating a lottery system for intensive care as hospitals become rapidly overwhelmed by cases.
The blueprint also warns of a "complete or partial collapse of some or all hospital infrastructures" brought on by a worst-case pandemic scenario of "catastrophic severity".
But Britain's top healthcare professionals have played down the warnings, saying the country's high state of readiness since fears over bird flu, as well as high levels of immunity, mean the impact will be less severe.
The predictions come in a report prepared by the Department of Health last September called the "Pandemic influenza: Surge capacity and prioritisation in health services".
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.