Pandemic flu has ranked as the biggest risk to the British public, according to a national threat assessment.
Such an incident would be the most damaging, claiming up to 750,000 lives.
The Cabinet Office's National Risk Register, part of Prime Minister Gordon Brown's overhaul of homeland security strategy, considers the likely dangers posed by threats including terrorism, climate change, extreme weather and pandemic disease.
It said the government has stockpiled enough doses of the antiviral drug oseltamivir, known as Tamiflu, to treat a quarter of the population.
"This should be sufficient to treat all those who fall ill in a pandemic of similar proportions to those that occurred in the 20th century," it said.
"The government is collaborating actively with international partners on prevention, detection and research, and is taking every practical step to ensure that the UK is prepared to limit the internal spread of a pandemic and to minimise health, economic and social harm as far as possible."
But a pandemic is only the fifth most likely type of risk, and is ranked as slightly less likely than a severe weather incident. Another targeted attack on the country's transport network, such as the July 7 incident, ranked as the number one threat.