Closed-circuit television (CCTV) system operators would need exceptional justification for recording sound as well as video, the Information Commissioner's Office (ICO) has warned.
In a newly revised CCTV code of practice the ICO said that sound recording is intrusive and unnecessary in most circumstances, and that the use of sound recording could undermine any public support there is for CCTV.
The UK has more CCTV cameras than any other country in the world according to the ICO. There are an estimated 4.5 million cameras in the country, recording the average person 300 times a day, according to some estimates.
The ICO has raised concerns about a trend for attaching microphones and sound recording equipment to cameras.
"CCTV must not be used to record conversations between members of the public as this is highly intrusive and unlikely to be justified," said the new guidance.
"You should choose a system without this facility if possible. If your system comes equipped with a sound recording facility then you should turn this off or disable it in some other way."
The ICO said that there are limited circumstances in which audio could be used, such as help systems where a person can initiate a conversation with an operator.
But some things should never be recorded, it said. "Conversations must not be recorded, and operators should not listen in." said the guidance.
"In the limited circumstances where audio recording is justified, signs must make it very clear that audio recording is being or may be carried out."
Assistant commissioner at the ICO Jonathan Bamford said that organisations must be careful in their use of cameras and recording equipment.
“[CCTV] can be extremely intrusive, monitoring ordinary individuals as they go about their day to day business," he said.
"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."
The ICO conducted a consultation exercise on the new guidance and has just published the updated code of practice.
It also conducted research into the attitudes of the public to audio recording.
It said that it found seven out of 10 people surveyed were opposed to the use of audio recording as part of CCTV systems.
It also found that almost half of those surveyed were not aware that the use of CCTV is covered by the Data Protection Act, one of the laws which the ICO is charged with enforcing.
The introduction of new rules may not do enough to make CCTV systems illegal. CCTV compliance consultant Bernie Brooks told OUT-LAW Radio last year that in his experience 95% of systems were operating outside of existing laws regarding notification of the collection of information and registering with the ICO.
"I would say that 95% are non-compliant in one way, shape, form or another with the [Data Protection] Act," he said. "Obviously that's quite a worrying thing.
If the system is non-compliant it could invalidate the usefulness of the evidence in a court of law."