Plans to record all conversations in taxis licensed by a city council are a "staggering invasion of privacy" which show a "total disregard for civil liberties", campaigners have said.

Oxford City Council plans to ensure all of its 600-plus cabs are fitted with at least one CCTV camera to record all conversations between passengers.

The civil liberties campaign group Big Brother Watch said it will complain to the Information Commissioner over the scheme, which will record all conversations from once the engine is running until 30 minutes after the ignition is switched off.

Nick Pickles, the campaign group's director, said: "This is a staggering invasion of privacy, being done with no evidence, no consultation and a total disregard for civil liberties.

"Big Brother now has big ears, and they are eavesdropping on your conversations with absolutely no justification. Given that one rail route to Witney is through Oxford, we'll be letting the Prime Minister know that his staff might want to avoid using Oxford cabs."

A spokeswoman for Oxford City Council said the new CCTV rules for taxis "would mean that video and audio would run all the time within the vehicle".

She said: "There are laws in place (data protection, human rights, CCTV code of practice) that require the viewing of such images to be necessary and proportionate, and therefore must relate to a specific complaint/incident/investigation.

"The officers are not permitted to view any images that do not relate to the actual matter being investigated. The risk of intrusion into private conversations has to be balanced against the interests of public safety, both of passengers and drivers."

She added that the footage will not be routinely viewed, but will be kept for 28 days on a CCTV hard-drive in case it is needed following a specific incident. The policy will also be kept under review. The council spokeswoman added that the cost of installing the CCTV system with one camera is about £400 and there are currently 107 black cabs and 545 private hire vehicles in the city.

A spokeswoman for the Information Commissioner's Office said: "CCTV must not be used to record conversations between members of the public as this is highly intrusive and unlikely to be justified." She added that audio recording may be justified, subject to sufficient safeguards, if the specific recording is triggered due to a specific threat, such as a panic button in a taxi cab.