Businesses that allow staff to run Skype PC-to-PC calls are putting their computer systems at risk.

Analyst Butler Group says Skype was not designed to work on an enterprise network, and when it does it often causes systems issues leaving companies vulnerable to 'super nodes'.

'Super nodes occur when a large number of PCs in an organisation are using Skype from behind a corporate firewall and need to find a route out onto the wider internet,' said Richard Edwards, principal analyst at Butler Group.  

'The super node provides just that, and acts as a router for directory searches, like finding other Skype users.'

The computer that becomes the super node then starts to suffer as its network segment becomes overwhelmed with Skype traffic.

People love the idea of high quality PC to PC calls for free, but there is no such thing as a free lunch, Edwards says.

'If people want to benefit from cheap phone calls in this way then they have to take their share of system load,' he said.