Some of the world's leading food manufacturers have begun marketing to children on social networking websites and internet chat programs.

Brands such as McDonald's, Starburst, Haribo and Skittles are using the internet to target children now that new rules from the media regulator Ofcom have made it difficult to advertise during children's television.

At the beginning of July, the sweet brand Skittles paid a six-figure sum to set up a profile on the social networking site Bebo which has already been viewed more than 50,000 times and attracted more than 3,500 "friends". In an interview with the Guardian, a Bebo spokesman described these "friends" as "brand ambassadors". Bebo users have to declare they are at least 13, but it is known that much younger children do use the site.

Such practices have raised concerns among MPs, who said yesterday the government's clampdown on advertising "junk food" on children's television had failed because it allowed brands to simply divert their budgets online.

In response to the findings, revealed in full in today's G2, the Department of Health said it had already expressed its concern to food and drink manufacturers and to advertisers about these practices and was "monitoring closely the change in the nature and balance of food advertising".

Advertising to children on the internet is a largely unregulated area. Ofcom's remit does not stretch that far, and although the Advertising Standards Authority's code of conduct was recently extended to include online marketing to children, it has left open a loophole that many brands exploit. That is that anything classed as "editorial" is exempt from the ASA code.

Here, editorial includes anything on a brand's own website, which is why the McDonald's site can feature a Kids Zone section, inviting children to play a Space Invaders-style game where they have to shoot down McDonald's logos.

McDonald's chief marketing officer for northern Europe, Jill McDonald, admitted to the Guardian that the company did not need to advertise to children to make a profit, but said it was using its marketing as a force for good.

As well as using social networking sites, some brands are also using internet chat programs such as Windows Live Messenger (formerly MSN messenger) to target potential customers, who are urged to download branded "buddies" to their friends list. These virtual buddies then give the users the lowdown on the brand's latest activity.

Nigel Evans, a Conservative MP and member of the select committee for culture, media and sport, which monitors new media and the advertising industry, said: "If the tobacco industry deployed those sort of tactics then no doubt there would be a change in legislation - government would clamp down on companies like a ton of bricks."

Philip Davies, a Conservative MP also on the select committee, said the Guardian's findings showed the impotence of the Ofcom regulations. "It was entirely predictable that this would happen. This was the folly of introducing a ban on so-called junk food advertising. It was a piece of gesture politics," he said.

Since April 1 this year, adverts for brands classed as being high in fat, salt or sugar have been banned in or around programmes made for children, or which are likely to appeal to children aged four to nine. This will be extended to programmes aimed at four- to 15-year-olds from January next year. Social-networking sites are used by more than 70% of young internet users and 41% of UK adult users, Ofcom said.