Conversations between passengers on buses could be snooped on by surveillance microphones under new plans exposed by drivers.
They have threatened to strike over the new technology which some fear could breach civil liberties.
However transport bosses insist the audio recorders are not switched on and said they have only trialled the system in the depot.
Workers union Unite, which represents bus drivers in Brighton and Hove, accused the city's bus company of spying on passengers and drivers.
Microphones have been installed in drivers' cabs on some of Brighton and Hove Buses' fleet. They are capable of listening in on conversations as far back as the seats on the bottom deck.
There are plans to use the technology to keep both drivers and passengers safe as antisocial behaviour on buses increases. It would provide evidence which could be presented to the police on top of existing CCTV footage.
Unite alleges the company has already used the technology in regular passenger service but the bus company denied this.
Regional officer for Unite Janet Nobbs said: "It is outrageous that Brighton and Hove Buses have begun recording not only our members' but also passengers' conversations without their knowledge or consent."
Buses already record a significant amount of data from the driver's cab, including from cameras and the buttons they press and when.
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The union claims the use of the new equipment only came to light when a member of management presented a recording to a Unite representative during a disciplinary case involving a driver.
Brighton and Hove Buses denied any recordings from inside the cab were presented in a disciplinary hearing.
Ms Nobbs added: "There are no warning signs on the buses and I think most of the public would be appalled to know that bus company executives could have access to private conversations.
"The bus company needs to explain itself - not just to Unite and the Brighton public but to the Information Commissioner's Office as well."
What does the law say?
It can be legal to monitor both staff and customers while in the workplace, including on public transport, given safeguards are put in place and they are told these recordings are being made.
Mike Cain, a lawyer at employment specialists Leigh Day told The Argus:
"The monitoring of employees through use of recording equipment or even the interception of their electronic communications is a common occurrence in the modern workplace and it cannot be argued that the practice is entirely unwarranted or always unlawful, most especially where the purpose is connected to safety and the prevention of criminal or violent or dangerous conduct in a way that protects employees or service users.
"The issue here is one of transparency, notification and where necessary consent. Staff and service users should be notified of the existence of the recordings and the policy reasons for the practice to rule out complaints based on the expectation of privacy.
"Where that notification does not happen, there is likely to be jeopardy for an employer in respect of breaches of Data Protection and the risk of complaints.
"When it comes to the use of recorded material in disciplinary proceedings, however, things are less straightforward and that fact that allegations of wrongdoing are supported by recorded evidence that was created without employee notice or consent does not necessarily or automatically mean that any disciplinary sanction such as dismissal will be unfair although it is of course a relevant factor for consideration.
"Each case will be circumstantially different and require specialist advice”
Data regulator the Information Commissioners Office said the capability to record audio should be switched off by default and only used in exceptional circumstances.
Guidance issued to data controllers, such as the bus company, says a trigger switch to use it only when it is needed should be considered due to its "intrusive nature".
A spokesman added: "Continuous activation requires strong justification that you need to document and risk assess thoroughly."
The union claims there are no warning signs on the buses and drivers did not know the microphones were installed.
The Argus understands the trial took place at the Newhaven depot, which received more than 30 new buses last month.
Susannah Copson, legal and policy officer at campaign group Big Brother Watch, said it was a "clear violation" of passengers' and bus drivers' right to privacy.
She added: "Members of the public should not feel like they are having their every word recorded while on a public bus. This move is entirely disproportionate and it's clear that the bus drivers themselves do not welcome it.
"Brighton and Hove Buses should reflect on the criticism directed at them from both unions and civil liberties groups and remove this surveillance tech from their buses."
Big Brother Watch has previously campaigned against police drone surveillance cameras used on Brighton beach and facial recognition technology becoming more prevalent on the high street.
Audio recording is already used by other bus operators in the UK including Stagecoach which runs the 700 and 701 buses in the city.
Managing director of Brighton and Hove Buses Ed Wills said: "The trial at our Newhaven depot took place in conjunction with our trade union to ensure the system worked.
"The safety and support of our colleagues is our priority and audio helps to remove any ambiguity if video footage ever needs to be viewed. If and when this is implemented, signage will be updated in advance of the system being used."
Stagecoach tells passengers in its conditions of carriage that video and sound recordings are "used solely for the monitoring of safety, security, service quality and in support of relevant criminal and civil legal proceedings and complaint investigation". It said there are signs where it is in use.
Some 1,000 bus drivers in the city are now being asked to ballot on industrial action over use of the technology.
Unite general secretary Sharon Graham said: "It is outrageous Big Brother-style behaviour from the bus company, who are secretly recording drivers and their passengers without warning.
“Such is the depth of anger over this our members are now discussing potential industrial action as the bus company has refused to discuss matters reasonably."
The Information Commissioner's Office said it has received concerns from Unite and is awaiting "formal notification" for it to be considered.
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