British cosmologist Stephen Hawking on Monday launched the biggest-ever search for intelligent extraterrestrial life in a $100-million (92-million-euro), 10-year project to scan the heavens.

Russian Silicon Valley entrepreneur Yuri Milner, who is funding the Breakthrough Listen initiative, said it would be the most intensive scientific search ever undertaken for signs of alien civilisation.

"In an infinite universe, there must be other occurrences of life," Hawking said at the launch event at the Royal Society science academy in London.

"Somewhere in the cosmos, perhaps, intelligent life may be watching.

"Either way, there is no bigger question. It's time to commit to finding the answer, to search for life beyond Earth.

"It is important for us to know if we are alone in the dark."

The project will use some of the biggest telescopes on Earth, searching far deeper into the universe than before for radio and laser signals.

"Breakthrough Listen takes the search for intelligent life in the universe to a completely new level," said Milner, a former physicist.

He said the scan would collect more data in one day than a year of any previous search, tracking the million closest stars, the centre of our Milky Way galaxy and the 100 closest galaxies.

Earth's telescopes would be able to detect a signal from similarly-advanced technology sent from the centre of the Milky Way.

"We don't need to assume that civilisation is way more developed than we are," Milner said.

- 'Huge gamble, colossal pay-off' -

Martin Rees, Britain's astronomer royal and one of the project leaders, said modern technology allowed much more sensitive searches than ever before, though he cautioned against expectations of finding intelligent alien life.

"It's a huge gamble, of course, but the pay-off would be so colossal... even if the chance of success is small," the astrophysicist said.

However, the possibility of finding life had effectively risen a billionfold through the identification of billions of Earth-like planets in the Milky Way, he said.

"Is there life out there? We may not answer it but this gives a bigger chance that it may be answered in our lifetime," he said.

The programme will be 50 times more sensitive than previous searches, and cover 10 times more of the sky, experts said.

It will scan at least five times more of the radio spectrum, and 100 times faster, while in tandem undertake the deepest and broadest-ever search for optical laser transmissions.

All the data will be publicly available, allowing anyone interested to do their own trawling.

- Debate over sending messages -

The project is allied with the Breakthrough Message initiative, a global competition with a maximum $1 million prize pot to come up with the best message humans could send into the void.

However, there is no commitment to send any such message, as experts are torn on the wisdom and ethics of doing so.

Frank Drake, who sent messages into space in the 1970s, said: "We know there are people who are afraid that sending is going to endanger us."

Hawking has warned in the past about the risks of making contact with other civilisations.

"A civilisation reading one of our messages could be billions of years ahead," the 73-year-old mastermind said.

"If so, they will be vastly more powerful and may not see us as any more valuable than we see bacteria."

However, Rees said: "I suggest they know we're here already."

Drake said it could take at least 200 years before even the possibility of a reply to "shot in the dark" messages beamed into space.

By waiting to find civilisation, we could then tailor a "useful message" to send them.

Ann Druyan, who sent music on the 1970s Voyager probe launches, said the first thing to establish would be a means of communication and an intention to learn.

"We'd want to know their history, social forms, how they understand the origins of the universe," she said, but as for a first message, "'Hello' would be right up there".