A phone app has sparked worries that outsourcing giant Mitie has the power to snoop on its workers.
Staff, hired for support jobs such as cleaning and security, were told to download its InTouch app on their own personal phones to clock on for shifts this week.
But fears were raised because the app requests access to location data and media files.
The RMT union, which represents Mitie staff working on railways, is considering legal action over the privacy concerns.
General secretary Mick Cash told members: “It is truly shocking the company has proposed this idea without any discussions. The union has serious concerns over privacy with tracking facilities.
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“The app will have an impact on people’s work-life balance and cause unnecessary stress at work.”
Mitie, which has 77,000 staff in the UK and Ireland and runs NHS and Whitehall contracts, told workers the app was needed to do checks during shifts, request holiday and see shift summaries.
But the RMT said it appeared to track a phone’s movements.
It claimed workers could be forced to send a photo to superiors to prove they are at work.
Workers have raised concerns about the app encroaching on their personal lives, and wanted to know how people without a smartphone would now cope.
A Mitie security guard in Warwickshire said: “I want my phone to be my phone. I don’t want it to be for work. Where does the line get drawn?
“If they want to do it then they should be issuing work phones.”
Mitie, whose chief executive Phil Bentley earns a £900,000 salary, only gives phones to area managers and above.
It said the app met data protection and privacy rules and staff with no smartphone could keep using an existing phone system.
A spokesman added the app would “make booking and managing shifts easier, while allowing us to ensure the personal safety of our colleagues working alone or at remote locations”.