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 ahead of a speech at the Royal United Services Institute in London, warned that an outbreak of a lethal respiratory virus such as smallpox would be more dangerous than even a nuclear attack.

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

"Bioterrorism is a much larger risk than a pandemic," he said. "All these advances in biology have made it far easier for a terrorist to recreate smallpox, which is a highly fatal pathogen, where there is essentially no immunity remaining at this point.

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"When you are thinking about things that could cause in excess of 10 million deaths, even something tragic like a nuclear weapons incident wouldn't get to that level."

Last year the Nuffield Council of Bioethics warned that "garage scientists" could unleash dangerous genetically modified organisms into the environment using unregulated technology which is available online.

Scientists are concerned that a new technique, called Crispr, is now so cheap and widely available that amateurs will start experimenting at home.

Gates said that today's widespread global travel meant that a future pandemic could be more deadly than the Spanish flu outbreak in 1918, which killed up to 100 million people.