The results - which Symantec calls the `dirtiest web sites of summer 2009' - show that 48% of the potentially bad sites contain adult content, whilst the other 52% are mostly sites selling other goods and services.

The most common reasons for the sites being marked as potentially bad are viruses, security risks and browser exploits.

Delving into the report reveals that the average number of threats per site is around 18 000, compared to average of just 23 for websites in general.

Symantec says one of the most interesting aspects of this, the first analysis of websites collated by its user base, is that 75% of the sites marked as bad have distributed malware for more than six months.

Commenting on the results, Rowan Trollope, Symantec's senior vice president of consumer, said that there has been exponential growth in the number of online threats that are constantly evolving as cybercriminals look for new ways to target your money, identity or assets.

"In 2008, most new infections occurred while people were surfing the web. Norton Safe Web provides visual ratings that let consumers know about potential risks before visiting a site", he said.

And armed with this information, he added, consumers can make informed decisions about which sites to visit.

According to Symantec, using Safe Web - which has went into beta testing earlier this month - users can differentiate safe websites from malicious ones by observing the visual site ratings delivered within search results from sites like Google, Yahoo and Live Search.

"Our goal for Norton Safe Web is to provide users with the most up-to-date and accurate website ratings of any offering on the market", said Trollope.

This is achieved, he explained, by leveraging the millions of members of Norton community watch, an internet `neighbourhood watch' scheme that relies on the eyes and ears of the Norton user base to detect and report trouble.