Doubleclick.net and Demon.co.uk were among the top users of so-called web bugs that secretly track where their visitors go on the web.

According to a report by internet security firm Security Space, Doubleclick.net was the top user of web bugs, followed by Akamai.net, LinkExchange.com, Bfast.com and Demon.co.uk.

Security Space scanned more than 100,000 active websites, or about four per cent of total internet sites, to find the bugs. Extreme-dm.com, HitBox.com, LinkSynergy.com, Akamaitech.net and Commission-Junction.com rounded out the top ten, while Netscape.com, GoTo.com and Weather.com are included in the top twenty five.

Many site operators place web bugs on their pages to collect information such as which pages are being read most often. The bugs can be matched with cookies, the electronic files that are stored on PCs that can contain personal information such as names and email addresses.

Advertisement networks, for example, can use web bugs to add information to a personal profile of what sites a person is visiting. The browser cookie of an ad network identifies the personal profile. In addition, another use of web bugs is to provide an independent accounting of how many people have visited a particular site.

Recently, US privacy advocacy, The Privacy Council, warned that web bugs could steal a computer user's entire email address book by simply clicking on a bugged web page.

"Through an insecurity in Windows, they showed how easy it is for people to get stuff off a consumer's hard drive," said Richard Smith, chief privacy officer at another privacy advocacy group, the Privacy Foundation.

The Privacy Foundation is testing a beta version of a browser plug-in, called a web bug detector, which allows people to identify the tags.