A new civil liberties controversy has flared up over the news that police chiefs are considering using high-powered microphones to "eavesdrop" - as critics will see it - on crowds at the London 2012 Olympics.

A high-ranking officer told the BBC the proposal to strengthen security by using microphones alongside closed-circuit TV involved "taking public surveillance to an entirely new level".

But the former home secretary David Blunkett called publicly on the government to block the scheme.

He told BBC Radio Five Live's Weekend News programme that the suggestion was "simply unacceptable", and smacked of the "surveillance state".

Mr Blunkett said the idea echoed the fictional authoritarian Brave New World of Aldous Huxley's novel.

"As you walk down the street you expect to be able to have a private conversation," he said.

"If you can't guarantee that - and here is someone speaking who has been pretty tough in terms of what should be available to protect society - I believe we have slipped over the edge."

He said he hoped the government would not authorise it.

"There is an enormous difference between surveilling people in terms of CCTV - where what you see is what anyone can see walking down the road - and actually recording someone's private conversations," he said.

Safeguards

The idea of using audio surveillance at the Olympics is being considered among proposals for enhancing security at the Games, well-placed sources have told Five Live.

The devices would be able to pick up conversations up to 100 yards away.

But details of how the system might be used have not been worked out.

Safeguards could be included in the scheme, such as requiring audio surveillance to be authorised by a senior officer and ensuring the system was reviewed by an independent monitor.

The proposal has not yet been finalised, however, or submitted for political approval.

Police sources concede that public debate and political approval would be needed before such a controversial extension of police powers could be approved.