A bioterrorist attack which could wipe out 30 million people is becoming increasingly likely because it is easier than ever to create and spread deadly pathogens, Bill Gates has warned.

The Microsoft founder, who was speaking before a speech at the Royal United Services Institute in London (RUSI), warned that an outbreak of a lethal respiratory virus such as smallpox would be more dangerous than even a nuclear attack.

Mr Gates, whose charitable foundation funds research into quickly spotting outbreaks, said it was more important than ever to help foreign countries monitor diseases to prevent a global tragedy.

“Bioterrorism is a much larger risk than a pandemic,” he said.