The Information Commissioner's Office (ICO) has warned companies that fitting microphones to CCTV cameras to record people's conversations is a "highly intrusive" development.

Sound recordings would only ever be justified in "highly exceptional circumstances", according to the data watchdog.

The warning comes as the ICO launched a new code of practice outlining the key issues that organisations and businesses must consider when routinely using CCTV equipment.

Last year Westminster Council trialled audio devices on six CCTV cameras, in Soho, London, which it said was to try out wireless networks.

However, the ICO said its research showed that seven out of 10 individuals oppose the idea of CCTV cameras which record their conversations. It said companies and organisations must not become so "intrusive" that they lose the public support the use of CCTV cameras currently have.

Jonathan Bamford, assistant commissioner at the ICO, said: “CCTV enjoys a lot of public support and can have benefits such as helping with the detection of crime.

"However, it can also be extremely intrusive, monitoring ordinary individuals as they go about their day-to-day business.

"It is essential that organisations and businesses use CCTV responsibly in order to maintain public trust and confidence in the use of CCTV and to prevent its use becoming increasingly viewed as part of the surveillance society.”

As well as advising organisations on how to use CCTV responsibly, the code of practice also sets out what they need to do to ensure people's rights are protected. According to the research, over half of individuals are not aware that the use of CCTV cameras is covered by the Data Protection Act.

The code also provides advice on the retention and use of CCTV images and outlines some of the circumstances when it would be appropriate to disclose images captured by CCTV, for example, to police for the investigation of a crime.