Civil liberties campaigners on Monday expressed outrage at an English city's plan to install audio recorders in licensed taxi cabs as a security measure.

Officials in Oxford say that starting next year its 600-plus cabs will carry audio-equipped cameras that run whenever the vehicle is in use, "leading the way" in ensuring the safety of passengers and drivers.

"This is a staggering invasion of privacy, being done with ... a total disregard for civil liberties," said Nick Pickles of Big Brother Watch. He said his group would complain to an independent regulator.

The Oxford officials didn't cite any crime figures regarding taxi drivers or their passengers to justify the decision.

Britain has hundreds of thousands of surveillance cameras in public places, including many trains, subways and buses.

But guidelines from independent regulator the Information Commissioner's Office say recording conversations is "highly intrusive and unlikely to be justified."

Oxford City Council dismissed concerns over privacy violations, saying the recordings of conversations between passengers would be available only to police or other authorities in connection with specific investigations.

"The risk of intrusion into private conversations has to be balanced against the interests of public safety, both of passengers and drivers," said a council spokeswoman, who declined to be identified, in keeping with departmental policy.

She added that the level of privacy passengers could expect in a taxi was "far lower" than in a home or personal vehicle.

Julian Alison, the licensing team leader for the city, said the recordings would boost the confidence of taxi-users and create a safer environment for drivers.

The council is "committed to ensuring the safety of those who live in, work in, and visit the city and through the implementation of schemes such as CCTV in licensed vehicles," Alison said.

Under the proposed plan, conversations in taxis will be recorded from the time an engine is running until 30 minutes after the car's ignition is turned off.

Most taxis would require one camera positioned above the rearview mirror, but the council said larger vehicles could require an extra camera to ensure the whole of the interior is viewable.

The footage recorded by the audio-equipped cameras will not be regularly viewed, but kept on a hard-drive for 28 days in case a specific investigation or incident calls for a viewing.