Personal data - information about someone who is identified - is protected under the Data Protection Act.
The act says the data should be used fairly, lawfully and transparently. It should be for a specified purpose and should not be kept for longer than necessary.
Before monitoring of employees starts, employers should ensure staff are aware, guidance from the Information Commissioner's Office (ICO) says. Staff should be told the reasons and how the information collected could be used.
The ICO also advises employers to consider the potential negative effects on staff and whether there are less intrusive alternatives.
Employment lawyer Max Winthrop says transparency is key. If an employer is using data gathered from monitoring workers - for example to find out how productive they are - as the basis for disciplinary action, this should be in the employees' contracts.
However, he says employees cannot necessarily expect the same level of privacy in the workplace, for example when using a work computer, as outside of work.
Trade union body the TUC has warned that things like monitoring toilet breaks and tracking workers' movements creates "fear and distrust".
It says any monitoring should be agreed with employees, not imposed upon them.
A 2018 survey by the TUC of more than 2,000 people found attitudes to workplace surveillance depended on its nature.
For example, tracking the location of company property, such as vehicles, laptops and phones was considered unacceptable by fewer than a quarter of those surveyed.
But the use of facial recognition software and monitoring social media use outside working hours was considered unacceptable by about three-quarters of employees.
The union says surveillance can sometimes have benefits for employees. For example, tracking the location of staff can give them greater security and recording of calls can give staff protection against customer complaints.
However, Edward Houghton, head of research at the Chartered Institute of Personnel and Development, says surveillance technology can "cause more harm than good" as employees may feel they are not trusted to do their work effectively.
Best startups generally come from somebody needing to scratch an itch.