England's Astronomer Royal says he believes aliens could exist, possibly as balloon-like creatures floating in dense atmospheres.

Professor Sir Martin Rees, a leader in understanding the birth and structure of the Universe, says he remains enthusiastic about the search for alien life.

But, in a speech on Thursday, he said that attempts to find them had suffered, in particular, from "flakey"' associations with UFOs.

Professor Rees also said we should stop transmitting messages to outer space and instead, listen for signals from super-intelligent computers in the form of strings of prime numbers or digits.

Astronomer Royal Prof Sir Martin Rees

Speaking on BBC Radio Cambridge he said: "You might find intelligent life so far away that signals take maybe 10 or 20 years [to get here] - so if we detect the life then we can listen for their signals.

"But there's no scope for snappy repartee, as it were, because to send a signal there and back would take decades."

Sir Martin believes life could exist in the form of balloon-like creatures the size of insects which might float freely in space.

His thoughts were made public in a speech to the Royal Society of Medicine (RSM) and the Royal Society for the encouragement of Arts, Manufactures and Commerce (RSA)

Other topics touched on what astronomers would be able to tell us 20 years from now, about life on planets in other solar systems and the long-range future for human life.

He also delved into the possibility of multiple universes, where "what we call the laws of nature would be no more than local bylaws".

Notwithstanding his comments, Professor Rees describes himself in general as "agnostic" as to whether life exists beyond the Earth's atmosphere.