Google made it official on Wednesday that it would start getting into 'behavioural targeting', which would gear ads to the users' interests and online behaviour no matter where they are browsing. The move ran into immediate opposition from privacy advocates, with Privacy International calling for a parliamentary inquiry.

The search giant is offering users the chance to see and edit their profiles and it will also offer them the choice to opt out of the service, but opponents say this is not enough. They feel Google should have given users the option of opting into rather than out of the service. They are also concerned that persons under 18 would be targeted.

The trial service launched on YouTube and Google on Tuesday, but advertisers will not be able to display advertisements until April. In the first couple of weeks of the program, which is in test mode, 20 to 50 advertisers approved by Google will run the ads, though the programme will roll out much more widely later this year.

Google's targeting involves placing people-or more accurately, their Web browser, minus personal information-into one or more of 600 categories, such as baseball fan or luxury car seeker.

The system uses a cookie - a small piece of text that lives inside a web browser - to track users as they visit different websites that show ads through its AdSense programme. Users will be put into categories based on the content of the pages they visit.

"If a user is a keen traveller and visits lots of travel sites, Google could show them more travel-related ads," the search giant said in a statement. "We believe that ads are a valuable source of information that can connect people to products, services and ideas that interest them. By making ads more relevant and improving the connection between advertisers and our users, we can create more value."

A spokesman said, "Google will not associate sensitive interest categories with your cookie - such as those based on race, religion, sexual orientation, health, or sensitive financial categories - and will not use these categories when showing you interest-based ads."

The online advertising industry is keen to push behavioural ads and, at the beginning of March, the UK-based Internet Advertising Bureau (IAB) published a code of practice that Google signed up to.

According to Forrester Research, 26 per cent of European online advertisers used behaviour-based advertising during 2008. The IAB estimates that it could generate an income of £200 million in the UK annually.

The ads, which Google is calling interest-based advertising, will launch in beta and give advertisers the opportunity to draw on Google's wide reaching network to target users based on their interests.

For the first time, Google will give users the ability to access and edit the interest categories it has compiled via its Ad Preferences tool, or opt out entirely.

Google will use cookies to track users across its content network and YouTube and serve ads based on assigned categories such as sports enthusiasts or car buyers.

The service will launch in beta in early April with a small number of advertisers in the US and Europe. Google aims to make the service widely available by the end of the year.

A Google statement said, "We think we can make online advertising even more relevant and useful by using additional information about the websites people visit.

"Over time we expect our ability to get the right ad in front of the right person at the right time to improve as we build interest categories with the help of our users and publishers."

Google's move into behavioural targeting has been rumoured for some time following Google's purchase of ad-serving company DoubleClick last year.

But the move could cause further friction with online publishers, who have expressed concerns about handing over user data and inventory to ad networks.

It comes one week after new media age revealed Google was among a group of key industry players who had partnered with the Internet Advertising Bureau to issue good-practice guidelines and launch consumer education for behavioural targeting in a bid to settle the furore over the practice.