The millions of tourists coming to London for the Olympics will place Britain at extreme risk of a deadly flu pandemic, according to research being released today.

Britain is ranked second in the world, after Singapore, in terms of the risk of an avian or swine flu outbreak spreading, according to a study of more than 200 countries by the risk analyst Maplecroft.

The researchers warn that the scale of the threat is vast. Citing previous warnings from the World Health Organisation, they write: "There is little pre-existing natural immunity to H5N1 infection in the human population. Should the virus improve its transmissibility, the entire human population could be vulnerable to infection."

Britain is at "medium risk" of a pandemic emerging, where avian or swine flu jumps the species barrier and can be spread from person to person.

But factors such as crowded cities, a growing population and the sheer volume of travel in and out of the UK provide ideal conditions for a virus to spread if it makes it to our shores.

Health officials are already on high alert in the run-up to the Olympics, with the Health Protection Agency having increased its surveillance work to detect any signs of a virus. It has set up a monitoring system for hospital admissions and patients presenting to GPs, to alert staff to the first signs of mass contagion.