A weaponised smallpox virus could have a devastating effect on the world's major cities, infecting thousands because of the growing numbers of people with suppressed immunity, researchers have warned.

Smallpox, which was eradicated globally in 1980, is classed by the United States government as a category A bioterrorism agent because of its potential to spread rapidly, infect swathes of the population and incite widespread fear and panic.

Researchers at the University of New South Wales have used a mathematical model to predict how many people would become infected and how many would die in bioterrorism attacks on New York and Sydney.

The data is likely to be scrutinised carefully by public health officials in Britain in the wake of the attack on Salisbury last week. Russia is known to hold samples of the smallpox virus.