Extra-terrestrial life is to come under the microscope as scientists and academics discuss its possible effect on humanity at a gathering in London.

The second day of the conference, organised by the Royal Society, the UK's national academy of science, will hear talks on human responses to extra-terrestrial life.

Lord Martin Rees, president of the Royal Society, will ask whether a microwave transmission from light years away will result in fear and pandemonium, or perhaps even delight.

Other speakers at the conference, entitled The Detection of Extra-terrestrial Life and the Consequences for Science and Society, will investigate the implications of the discovery of alien life for religion and theology.

And the world's leading ET hunter, Dr Frank Drake, will discuss the ongoing search for beings from another realm.

Dr Drake said satellite TV and the digital revolution was making humanity more and more invisible to inquisitive aliens on other planets.

Dr Drake, who founded the SETI (Search for Extra-terrestrial Intelligence) organisation in the US 50 years ago, said the digital age was effectively gagging the Earth by cutting the transmission of TV and radio signals into space.

At present, the Earth is surrounded by a 50 light year-wide "shell" of radiation from analogue TV, radio and radar transmissions. But although the signals had spread far enough to reach many nearby star systems, they were rapidly vanishing before the march of digital technology. Dr Drake said to a race of observing aliens, digital TV signals would look like noise.

Any alien civilisations that existed were likely to be far more advanced than ours, he said. Their analogue TV age probably came and went long ago, before humans even thought about searching the sky for signs of intelligent life.

Dr Drake said he remained convinced that intelligent life existed beyond the Earth, despite the lack of success of SETI scientists who have spent half a century tuning into the stars.