The security backlash against Facebook has taken a new twist with a survey finding that large numbers of employees are now being blocked from accessing it.

The statistic comes out of a new global poll of 600 workers carried out by security company Sophos, which found that 43 percent of them were unable to access the site, while another seven percent could access it only with restrictions.

Extraordinarily, of the half who could access the social networking site, 8 percent stated that the access was allowed because companies feared an employee backlash if it was stopped.

The main worry appears to be that employees will waste time on Facebook, the latest in a long line of social networking sites to garner a mainstream following. However, in a separate poll carried out by Sophos, two thirds said that they were concerned about co-workers sharing too much information on the site, fearing this might lead to targeted phishing attacks against employers.

"More businesses are restricting access to these kinds of sites. Employees may not like it, but these websites can represent a security risk if used carelessly. Unless there's a work purpose, many firms do not see any reason why staff should need to access them during work time," said Graham Cluley of Sophos.

Companies known to have blocked the site reportedly included LloydsTSB, Credit Suisse, and Goldman Sachs. Sophos, for one, has made targeted blocking of instant messaging, gaming, social networking websites, P2P and non-allowed VoIP clients, one of the main features of its client security software.

"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," said Cluley.

In recent weeks, the information theft peril presented by Facebook has started to receive publicity, with Sophos leading the campaign. Facebook has also its own problems with security.

Sophos has published a security-cum-marketing guide to Facebook on its website.