A deal between McDonald's and Microsoft to produce a branded version of the popular MSN Messenger program containing advertisements for special offers at the hamburger chain has prompted complaints from consumer groups and MPs. They claim the so-called Theme Pack pushes junk food advertising to young people when Britain is struggling to deal with childhood obesity. They are also concerned that advertisers are turning to the internet and mobile phones to reach beyond parental controls to target children.

"We are concerned about the way that foods high in fat, sugar and salt are marketed to children," said Michelle Smyth of Which?, the consumer watchdog. "The McDonald's deal with MSN Messenger shows that those ways are becoming ever more imaginative and innovative."

MSN Messenger is one of the world's most popular instant messaging programs, and is regularly used by more than 10 million people in Britain. Although MSN does not allow advertisers to target particular audiences, it does boast an estimated 800,000 users under the age of 18. A Guardian/ICM survey last year showed that young people predominantly used the internet to communicate with one another via messaging software, chatrooms and email. On average, they spend around five hours a week doing so.

A McDonald's spokeswoman said that the company was not the only advertiser to use instant messaging. She said: "We are one of a significant number of companies offering Theme Packs on MSN. The Theme Packs are intentionally developed on an 'opt in' basis, where the user must deliberately select the icon themselves."

After so much bad press surrounding junk food the company is trying desperately to restore its image by giving more prominence to healthy eating options, boosting its advertising spend and sealing sponsorship deals with groups such as the England football team.

Critics say the move is a further signal that advertisers are moving outside traditional channels to find target audiences. Ofcom, Britain's media regulator, is expected to announce new curbs on TV food advertising to children later this month.

The popularity of instant messaging has given rise to the term spim, a close relative of unwanted email spam. Unscrupulous advertisers have begun using the medium, often disguising their marketing messages as genuine conversations from strangers.

"I think other people will attempt to use new media to reach audiences," said Michael Nutley, the editor of New Media Age magazine. "If you're targeting young people, online is an obvious way to do it. But there has been a lot of movement recently to start applying the offline rules to online - regulators are finding that they're having to look more at this stuff, because people are complaining about it."