Many of Germany's top companies are blocking access to Facebook and other social networking sites over fears of industrial espionage and other security concerns, according to a report released Sunday.

Business weekly Wirtschaftswoche said in an advance copy of its Monday issue that many companies on the Dax-30 blue-chip index saw an unacceptable risk posed by employees using such sites at work.

"Many external social media sites are no longer available to most of our staff due to security concerns," said a representative for the second-biggest German bank, Commerzbank.

Construction materials group HeidelbergCement drew similar conclusions before outlawing Facebook and micro-blogging site Twitter, and automaker Volkswagen said it had also banned "various social networks".

Beyond the risk of staff revealing trade secrets in online chats, companies fear that such sites could expose them to computer viruses transmitted by clicking on a link within someone's 'wall', according to the report.

"Before it was email that was the favourite gateway for damaging software -- today it is social networks," Christian Fuchs of anti-virus provider Kaspersky was quoted as saying.

Luxury car maker Porsche recently restricted Facebook use over industrial espionage fears, according to the report, while energy group E.ON and industrial gas giant Linde have also curbed access to Facebook and video sharing site YouTube at some of their offices.

Worries over lost productivity prompted other corporations such as Daimler to pull the plug on Facebook for some of its staff.

A study by IT security firm Clearswift indicated that 30 percent of German companies fear social networking sites will distract their employees from work if they have unlimited access.

However, a much larger group -- 56 percent -- cited security concerns as the primary reason to restrict such sites, Wirtschaftswoche said.