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The Trade Union Congress conceded that employers do have a legal right to stop staff using the sites at work, but suggested that a total ban is often an "over reaction".

The comment comes after several high-profile companies blocked access to Facebook, including Tesco, LloydsTSB, Credit Suisse, the Metropolitan Police, Camden Council, and Transport for London.

In a briefing published , called "Facing up to Facebook", the TUC suggested that workers should be allowed to contact friends during their lunch break.

Employers are urged to set out a social networking code of conduct, because ignoring the issue makes "the UK's Facebook users 3.5 million HR accidents waiting to happen".

Brendan Barber, TUC General Secretary, 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."

The TUC also warned that bosses using Facebook as a recruitment tool, whereby potential candidates are vetted online before being invited to interview, is an increasing problem and could result in legal action.