Fifty per cent of employees are blocked from accessing social networking site Facebook by employers who are worried that it saps productivity and is a security threat.

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

Graham Cluely, a senior technology consultant at IT security firm Sophos, that conducted the research, 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."

According to Sophos, a large number of Facebook profile pages contain users' current employment details, which could be used together with other stolen information by cybercriminals bent on committing corporate fraud, or to infiltrate company networks.

Last week, Sophos published research showing that 41 per cent of Facebook users were prepared to divulge personal information to a complete stranger (a small plastic frog called Freddi Staur), highlighting the extent of the problem facing businesses.

"Companies need to make their own mind up as to whether they want to allow their users to access websites like Facebook and MySpace during office hours. If workers are allowed to be given access to these sites then it's imperative that they are taught best practices to ensure that they are not putting their personal and corporate data at risk. Five minutes spent learning the ins-and-outs of Facebook's privacy settings, for instance, could save a lot of heartache later," said Cluely.