Google users will be given advance warning of potentially dangerous websites.

The search engine will divert users to a special Google page if a site is flagged up as containing malware such as Trojans or spyware. The special page states: "Warning – the site you are about to visit may harm your computer!".

The warning suggests that people try a different site but if they want to continue to the potentially dangerous webpage Google will not stop them.

Google is just one of a number of companies which have set up such alerts to warn surfers. Many of these sites offer popular free software such as screensavers and wallpapers. The Scandoo application from security company ScanSafe is a search service available free to anyone on the web.

Currently in initial beta testing, the aim is to provide an early warning system so that users can avoid the risk of clicking on unknown web sites.

The application doesn’t change the user's search engine but the initial query is typed into the Scandoo toolbar. Icons are then used to denote whether a site is safe or not.

A green tick indicates the site and its content has been verified, an amber question mark shows the site is so far unclassified. An image of a creepy-crawly indicates that a website could be dangerous.

Then users can also scroll over an icon that appears next to a search to see more detailed information about a site's content.

A similar security tool was launched at the end of 2004 by another security company GeoTrust. Its Trustwatch toolbar  ,which works with IE 5.01 or later, Firefox and Flock, was launched to help consumers avoid the growing number of phishing sites.

It also checks the security standards of websites that are conducting e-busi ness or requesting confidential information.

The list of malware websites for the Google service is supplied by the Stop Badware Coalition, an online Neighbourhood watch scheme. This organisation was founded last January by the universities of Oxford and Harvard.

The initiative is funded by IT vendors including Google, Lenovo and Sun Microsystems.

The Google alert page offers a link to the StopBadware.org website as well as the actual website the user was trying to visit.

Google said the current number of websites that will be flagged up is small. But the Google alerts will be expanded over time as more sites are visited by Stop Badware researchers.

This means the warning page will also have more detailed reports on a suspicious site and how the malware tries to compromise a person's PC.

These will be published as "user-friendly" reports on downloads Stop Badware has identified as malicious, as well as more detailed academic studies on the problem of malicious code.

It also solicits and publishes horror stories from web users who have been adversely affected by malware.