Employees should have access to social networking websites such as Facebook during office hours, the TUC has said.
The TUC wants guidelines for using the sites to be introduced
Some firms have blocked workers' access to the sites, or disciplined staff for misuse of the internet.
However, the union organisation says it is unreasonable to try to stop staff from having a life outside work and suggests setting guidelines instead.
Employment Law Advisory Services, which provides advice for employers, said access should be for business use only.
The TUC said employers were entitled to stop people using the sites during the working day but there should not be a total ban.
Staff should be able to use their time during lunch breaks to contact friends on sites such as Facebook, MySpace and Bebo, it added.
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's unreasonable for employers to try to stop their staff from having a life outside work, just because they can't get their heads around the technology.
"Better to invest a little time in working out sensible conduct guidelines, so that there don't need to be any nasty surprises for staff or employers."
But Peter Mooney, of Employment Law Advisory Services, said the TUC's call was "nonsense".
"Why should employers pay for the privilege of allowing their employees to access Facebook, MySpace or Bebo from work computers whether in an employee's lunch-time or not?
"The lunch break may not be a paid break but there is still a cost. As we all know, lunch-time spreads into work time, so where do you draw the line?
"Employers do not prevent employees from having a life outside work - which means after work when employees are quite free to access all the websites they wish from their own computers."
Access to the internet at work was "not an extension of playtime", he added.
Chris Reed from corporate communications company Fishburn Hedges told BBC News that Facebook helped keep his workforce happy.
"We let people exercise their own discretion and make people responsible for their own actions. That seems to work very well," he said.
In one case, Kent County Council banned its 32,000 employees from using Facebook.
The local authority said the move was an effort to reduce time-wasting.