Half of companies block staff access to Facebook because of fears about the social networking site's impact on productivity and security, new research reveals.

A Sophos poll of 600 workers found that 43 per cent were unable to access Facebook at work, while an additional seven per cent reported that use of the site was restricted.

In contrast, half of respondents said that their company did not block access to Facebook, with eight per cent specifying that the reason was 'fear of employee backlash'.

Sophos warned that a large number of Facebook profile pages contain users' current employment details, which could be used by cyber-criminals to commit corporate fraud or infiltrate company networks.

"Companies are split on the question of Facebook. Some believe it to be a procrastinator's paradise which can lead to identity theft if users are careless," said Graham Cluley, senior technology consultant at Sophos.

"Others either view it as a valuable networking tool for workers or are too nervous of a backlash if the site is suddenly blocked.

"Companies need to make up their own mind as to whether they want to allow their users to access websites like Facebook and MySpace during office hours.

"If workers are allowed access to these sites it is imperative that they are taught best practice to ensure that they are not putting their personal and corporate data at risk."

LloydsTSB, Credit Suisse and Goldman Sachs are among the companies reported to have blocked employees visiting Facebook.