A government risk assessment has warned that a flu pandemic would pose a greater threat to Britain than terrorism.
The Cabinet Office has published the National Risk Register which compares the impact of a range of incidents and gives new information on the chances that they might take place.
Pandemic influenza is thought to represent the greatest threat because officials believe that it is a question of "when, not if" it happens. Such a pandemic could claim between 50,000 and 750,000 lives.
"Influenza pandemics are natural phenomena that have occurred from time to time for centuries – including three times in the last century," the register's authors say. "The symptoms are similar to those of seasonal influenza but may be significantly more severe.
"Influenza pandemics arise as a result of a new influenza virus that is markedly different from recently circulating influenza viruses and therefore to which few people, if any, have immunity."
Pandemics have significant global human health consequences because of the speed with which they spread from person to person. "In addition to the severe health effects, a pandemic is also likely to cause significant wider social and economic damage and disruption," the report adds.