Small firms could be putting their businesses and employees at risk by not blocking access to social networking sites such as MySpace and Facebook.

Figures from security firm McAfee Avert Lab found that just one fifth of companies ban their employees from using the sites - despite them increasingly being used by opportunistic fraudsters to gather personal information.

Business networks are also being put at risk from viruses and spam by allowing employees to log onto social networking sites.

The survey of 1,000 IT professionals found that firms were also not taking into account the security risks posed by instant messaging and web-based email addresses.

Toralv Dirro, security strategist for McAfee, told Computer Weekly: "IT professionals often have to choose between what they know are serious security threats and other business priorities such as productivity and employee morale.

"We see usage policies used more often to bridge the gap between what IT departments would like to block and what is practical."

This week it was revealed that soldiers have been warned not to use social networking site for fear terrorists may target their friends or families.