Imagine a burger bar where most of the customers are under 18 and visit without their parents. There's a non-stop stream of burgers, fries and fizzy drinks for free. And the man behind the counter trills that a burger is "the food of the Gods".

Nothing is branded, but ask for a drink and you're told to "Obey your thirst" - the instantly recognisable marketing slogan for Sprite.

This place actually exists and, not surprisingly, it's wildly popular with burger-loving teenagers. It's not in the real world, thankfully, but online in Habbo, a virtual chat forum for teenage children.

Founded by a Finnish company in 2000, it has seen over 78 million visitors around the world, and receives seven million visitors each month. Ninety per cent of those are below the age of 18.

Users create their own little characters, called Habbos, which can explore public areas, such as a beach, restaurants, or cafes, and set up their own rooms. They can meet other young people under the watchful eye of moderators who make sure nothing untoward is going on.

It's an attractive platform for advertisers; music label EMI is currently promoting its Now 67 album, the latest in a long-running series of pop compilations. Music is fairly uncontroversial, but other things companies want to sell to teens are not, especially unhealthy food.

Worldwide, Habbo has run campaigns for numerous makers of what many would consider to be junk food. In the US, for example, it's running a campaign for a new snack called Cheez-it Stix and in July Habbo ran a campaign for Fanta in the UK.

This is a curious grey area in advertising regulations, where things that would not be allowed on television are perfectly legal.

Some things could never feasibly be controlled. It's hard to see how companies could be stopped from promoting whatever they like on their own sites, and pages hosted abroad, where rules don't apply, will always easily accessible from Britain.

But there are areas that do fall under the Advertising Standards Authority's remit: paid-for advertising on third-party platforms, such as Habbo. But the rules on advertising high fat, sugar and salt foods on television don't apply to paid-for advertising space online.

ince the Fanta campaign closed, Habbo hasn't run any campaigns marketing junk food, but there are plenty of messages that subtly imply that burgers and fizzy drinks are a good thing.

The site includes an area called the Habburger bar. It's a room made up to look like a normal branch of a fast food restaurant.

Go to the counter, and there's a bloke in a Santa hat called Phillip, who responds to requests for food. Ask for a Fanta, say, or a Coke, and he replies "Obey Your Thirst! Obey your thirst" - the advertising slogan of Sprite. Could this be a subtle subliminal marketing campaign?

Apparently not. Phillip is not a real person, but a "chatbot" - a character who appears to be real but is actually just delivering computerised responses.

A spokesperson for Habbo UK assured Channel 4 News that the "Obey your thirst" line was a total coincidence:

"It is not our intention to promote a fast food brand, or any brand, within these replies and the programmed responses are currently under review to ensure they are not misleading," she said.

A week later, Phillip had been reprogrammed. He now greets visitors with the words "Help yourself to a straw".

However, the place hardly promotes a healthy eating message. Ask for salad, and he replies, "do you want that with or without fries?"

I said "without" but was given fries anyway. And no salad. And if you don't want burgers there's a pizza place next door.

The site's spokeswoman said the burger restaurant was just part of the process of making Habbo look like the outside world as teenagers experience it.

"Habbo teens ... come to explore and interact with the engaging online virtual world that is similar to theirs offline."

So this may not be part of a subliminal marketing campaign for Sprite but there is some well-disguised marketing going on in there. Ask for a bar of chocolate in another part of Habbo - the Ice café - and you'll be offered fairtrade chocolate.

This is part of a marketing campaign by the Divine chocolate company. They invited kids to go into the Ice Café area and shout "stock the fairtrade choc" - as detailed on the Dubble website. As a result, Habbo now stocks fair-trade chocolate.

Virtual burgers don't make children fat, of course, and however boring the summer holidays may get, there can't be many kids who would choose to spend them eating digital salad.

But virtual habits will feed over into real world behaviour - at least that's what Sulake, Habbo's parent corporation, says on its website.

Habbo, it says, "offers an innovative and cost effective way to communicate and interact with the teen demographic, build brand loyalty and modify consumer behavior.

"Habbo pushes traditional online marketing campaigns into a virtual, activity and identity driven world, where the advertisers have the opportunity to become content providers."

It does seem to be a curious loophole - that advertisers can do on the internet what they can't do on television. As virtual worlds for children grow in popularity, is it one that should remain open?