Scientific progress is almost certain to bring disaster to planet Earth within the next few thousand years, according to top cosmologist Professor Stephen Hawking.
He predicts that as new technologies are developed, the number of threats to the human race will increase until some kind of global cataclysm is virtually inevitable.
In the time period before humans manage to escape to the stars, they will have to be "very careful", he says.
Prof Hawking's warning came during a question and answer session at the end of this year's BBC Reith Lectures, which he delivered to an invited audience at London's Royal Institution on January 7.
The theme of his talk was black holes, places where matter collapses under the pull of gravity to a point where the normal laws of physics break down.
Prof Hawking described black holes as "stranger than anything dreamed up by science fiction writers".
But his prophesy of doom came during the Q&A when a member of the audience asked him: "Do you think the world will end naturally or will man destroy it first?"
The professor, research director at Cambridge University's Department of Applied Mathematics and Theoretical Physics, replied: "We face a number of threats to our survival from nuclear war, catastrophic global warming, and genetically engineered viruses.
"The number is likely to increase in the future, with the development of new technologies, and new ways things can go wrong.
"Although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or 10,000 years.
"By that time, we should have spread out into space, and to other stars, so a disaster on Earth would not mean the end of the human race.
"However, we will not establish self-sustaining colonies in space for at least the next hundred years, so we have to be very careful in this period."
Most of the threats the human race faces come from progress in science and technology, he said.
He added: "We are not going to stop making progress, or reverse it, so we have to recognise the dangers and control them. I'm an optimist, and I believe we can."
On previous occasions, Prof Hawking has highlighted the potential risk of humans being wiped out by their own artificially intelligent creations, and the dangers of contacting highly advanced aliens.
Prof Hawking's talks will be broadcast at 9am on BBC Radio 4 on January 26 and February 2.