Research by the TUC showed that two out of three employees believe the trend fuels distrust and discrimination and could be used to set unfair targets.
A survey of 2,100 adults found that monitoring included checking internet use, recording time away from work duties and even the use of facial recognition software to assess workers' moods.
A third of those polled believe their social media activity is being "snooped on" when they are not at work.
Most workers believe employers should reach agreement and be legally required to consult staff before using surveillance.
TUC general secretary Frances O'Grady said: "Employers must not use tech to control and micromanage their staff.
"Monitoring toilet breaks, tracking every movement and snooping on staff outside of working hours creates fear and distrust, and it undermines morale.
"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.
"Unions can negotiate agreements that safeguard workers' privacy while still making sure the job gets done, but the law needs to change too, so that workers are better protected against excessive and intrusive surveillance."
The TUC highlighted the example of in-cab cameras constantly watching lorry drivers, even if they have a break.