Up to 1.2 million people could be hospitalised and 750,000 killed if the swine flu pandemic sweeps UK, according to draft Government guidance about an outbreak.
As patients are "rapidly" overwhelming hospitals, doctors may have to begin operating a lottery system for intensive care, The Mirror quotes the blueprint, as saying.
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.