Workers who spend too much time on the social networking websites such as Facebook are costing firms more than £130 million a day, it was claimed.
Employment law firm Peninsula estimated 233 million hours are lost every month by employees "wasting time" on the internet.
Mike Huss of Peninsula said: "Why should employers allow their workers to waste two hours a day on Facebook when they are being paid to do a job? Some companies are happy to let their workers use the internet for personal use, assuming that goals and targets are achieved.
"However loss of productivity through social networks such as Facebook is proving to be a major headache and my advice would be for companies to block access."
The study was based on a survey of 3,500 companies across the UK.
The TUC called last month for workers to be allowed to use social networking sites during office hours. More than three million people in the UK are believed to use Facebook and some employers have already taken steps to limit or bar access to the site.
Kent County Council has banned its 32,000 employees from using Facebook in an effort to reduce time-wasting.