More than half (56%) of UK workers believe they are monitored by their boss at work, research from the TUC claims.
The union said this could take a variety of forms, including monitoring internet use, tracking people's locations and timing toilet breaks.
It said workers feared surveillance data was being used to set unfair targets and "take away autonomy".
TUC boss Frances O'Grady said it threatened to undermine morale.
"Monitoring toilet breaks, tracking every movement and snooping on staff outside of working hours creates fear and distrust," she said.
"New technologies should not be used to whittle away our right to privacy, even when we're at work. Employers should discuss and agree workplace monitoring policies with their workforces - not impose them upon them."