The Trades Union Congress and the Industrial Society are calling on employers to draw up sensible guidelines on monitoring staff at work.
Employers should adopt codes of practice that encourage responsible behaviour, good management practice, and safeguard worker privacy, the organisations said last week.
"While employers have legitimate interests to protect, over-zealous monitoring can undermine employees' dignity and autonomy within the workplace," said Will Hutton, chief executive of the Industrial Society .
The organisations said polices should warn users that e-mails may be scanned electronically for obscene, indecent, racist or illegal remarks. But they should also allow reasonable personal use of e-mail, as long as this does not interfere with an employee's work.
Employers should also give assurances that union communications will not be monitored. And they should remind employees that their e-mails may be checked by others if they are away from work.