Employers are thought to block access because of concerns about the social networking site's impact on productivity and security within firms.

Sophos believes that firms think Facebook users' current employment details could be used by cyber criminals to commit fraud or hack into company networks.

A Sophos spokesman said:

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