Only a third of IT managers block access to music download sites, even though almost two-thirds perceive such websites to affect productivity and pose a possible security threat.

Music download and dating sites were blocked by just 36 per cent of companies, despite being rated 'dangerous' by 58 and 56 per cent of firms respectively.

"IT decision makers often have the difficult job of making a choice between what they know are serious security threats and other business priorities such as productivity and employee morale," said Toralv Dirro, security strategist at McAfee Avert Labs.

"This is where we see usage policies coming into play as a way to bridge the gap between what IT departments would like to block in an ideal world and what is functionally realistic."

Instant messaging applications were also marked out by 53 per cent of IT managers as being a danger to the company, while social networking sites rated 48 per cent and web-based email clocked up 46 per cent.

However, only around a quarter of firms blocked access to these sites or programs.

The McAfee findings also showed that only one in five businesses in Europe blocks access to social networking sites such as Facebook and MySpace.

This mirrors a study by Gartner, which found that enterprises are having difficulty coping with the rise of social networking sites.

The McAfee survey was carried out by ICM Research across 1,049 IT professionals in the UK, France, Germany, Italy, The Netherlands, Poland, Spain, Sweden, Austria and Switzerland.