In guidance available on WorkSmart, its working-life website, the TUC advises employers that they should have in place policies covering the use of e-mail and the web, including social networking sites.
The TUC advice suggests that although employers are completely within their rights to forbid staff from using sites such as Facebook, MySpace or Bebo in work time, "a total ban may be something of an over-reaction".
Instead, the TUC suggests that "sensible employers, realising that their staff spend many of their waking hours in work and lead busy lives, should be trusted to spend a few minutes of their lunch break 'poking' their friends or making plans for outside work".
The guidance accepts that employees are paid to do a job, and that it is clearly not acceptable for someone to spend hours a day on social networking sites when they should be getting on with their work. However, the TUC says, policies drawn up with the involvement of staff can set out what will be and what will not be allowed.
The TUC says not enough workplaces are being upfront about what they expect from staff in terms of personal conduct when using social networking sites. As a result, a number of employers have disciplined staff for their conduct online, and more cases are likely to follow "unless some sensible precautions are taken".
TUC general secretary, Brendan Barber, said, "Simply cracking down on use of new web tools like Facebook is not a sensible solution to a problem, which is only going to get bigger.
"It is unreasonable for employers to try to stop their staff from having a life outside work, just because they cannot get their heads around the technology. It is better to invest a little time in working out sensible conduct guidelines, so there are not any nasty surprises for staff or employers."