In two decades, child obesity has doubled. In the past year, £25m of TV food adverts were aimed at children. Is there a link? Depends who you ask, says Caroline Stacey

"Frosted Shreddies." My daughter is reading aloud to me from the Beano. I should be pleased her reading is coming along, but it's not quite what I had in mind. She's dictating over the phone as if to an imbecile (a tone familiar, I hope, to other parents), the name of the breakfast cereal she wants me to buy on the way home from work. Helpfully for the readers of the Beano, who we can probably assume are mostly under 12, there is a full-page advert from the cereal's manufacturer, Nestlé, on the page opposite Dennis the Menace.

There's a law against pester power. Adverts aimed at children must not explicitly encourage the little pests to nag you into buying something. You think an ad in a kid's mag, for food that appeals mainly to children but is bought by parents, counts as encouragement? It doesn't. According to the Advertising Standards Authority (which monitors all non-TV adverts), objecting to a product, and who it appeals to, is not a reason for objecting to an advertisement. And it's not in the ASA's remit to consider where they are placed.

"Frosted" is a euphemism for a Bob the Builder-style truckload of sugar. These little brown waffles of malty cereal are so encrusted with sugar they're almost white. It's the second ingredient, after wholegrain wheat and rice. Each 100g has 366 calories and 36.3g of sugar. My children are quite capable of eating 150g at a sitting. That's more than 500 calories, a third of the total a six-year-old girl should have a day.

Cereals like Frosted Shreddies and Sugar Puffs (glistening with a whopping 49 per cent sugar content), confectionary, savoury snacks, soft drinks and fast foods are the types of grub constantly wheedling their way into children's hearts and minds via lovable characters and catchy jingles. A whopping 90 per cent of food advertised in the gaps between children's TV programmes is high in at least one of the unhealthy trinity: sugar, salt and fat.

There is no legal definition of "high-fat", but experts accept that more than 20 per cent is a lot, and that more than 10 per cent sugar is high. As for salt, which contributes to high blood pressure and heart disease, more than 0.5g of sodium (that's the salt to watch out for) per 100g is not good, and processed foods are one of the main sources of it. One third of a packet of Walkers crisps, so successfully advertised by the ex-footballer and sports TV presenter Gary Lineker, is made up entirely of fat, and sodium content is 0.7g per 100g. Nestlé, which produces Cookie Crisps, KitKat, Aero, Munch Bunch Squeezy and Herta Dinky Dogs, is one of the top-spending food advertisers, along with Cadbury, Coca-Cola, McDonald's, Heinz and Pepsi (which owns Walkers and Quaker). Many have stars such as Lineker and Justin Timberlake on their side.

The term junk is generally applied to energy-dense, low-nutrient foods. Weight for weight, fat contains twice the number of calories as protein or carbohydrate, so you don't need to eat as much to start gaining weight. And the nation's increasing weight is getting out of hand. Half the adults in Britain are overweight, one in five is obese. As they put it on Radio 4's Today programme recently, "a tidal wave of obesity is heading towards us". The reality is a public health catastrophe. Stars like Britney Spears (a Pepsi-promoter) should be worried. How long is it before someone points a fat finger at her and says, "Britney made me drink it"?

When those adults spared regular contact with children come across a breaded turkey dinosaur (28 per cent turkey), they are horrified by what seem like caricatures masquerading as food. Nothing surprises the food-battle-weary parent, but we have strategies for keeping contact with enemy products at a minimum. Eventually we hope our progeny will lose interest in them. But for the sake of research I sacrificed the children in a sortie to the supermarket.

I dragged my two, 6 and 7, and another, 9, to Sainsbury's to help me identify the foods that tempt them. They were to show me the products they want, explain why, and how they knew about them. Then be told they couldn't have them. This may be good for their health, but it's not an activity recommended in childcare manuals. Worse, it was Health Week at school and they were in the middle of a full and inspiring programme about nutrition and exercise. One child had arrived at school in tears because he was worried he wouldn't be allowed to eat the white-bread sandwich, chocolate and crisps his mother had packed for lunch, but the message had been getting through. Until we got to Sainsbury's.

Once past the sweets and fags at the entrance, the rack of Finding Nemo videos primed them for yet more Nemo merchandising to come. But before that there was brand-free fruit and vegetables to get through. No danger of pester power there.

"I like Cheestrings, but the ads are rubbish." Does that show they're impervious to advertising? Perhaps now the agency's creative team will be sacked and a new one brought in to rethink the strategy. In the yoghurt aisle my guinea pigs were drawn to the cartons of Cadbury's Buttons and Flake mingling with the fruit flavours as if they had a similar nutritional virtue. And Nestlé's Nemo-covered fromage frais (nearly 16 per cent sugar). We compromised on Muller Corner's vanilla choco balls flavour, sweetened with fructose and dextrose, and with no sugar content given. (This was Health Week remember - it goes down in their food diary as yoghurt.) Their eyes lit up at the California-style Sunny D (15 per cent juice) on the other side of the aisle. However often they've been told it isn't orange juice like its neighbours, how do you convince them when its presented as one of the chilled fruit juice gang.

"This is the yummy section," my son said, approaching the cheesey snacky things he so longs to have a lunchbox to fill with. "I know these, I love that. I really like these pizza-making things," his friend said of Dairylea Lunchable Stack 'Em, Yummy Ham version (1g of sodium per 110g), which the child constructs by stacking slices of pizza-flavoured cheese and processed ham on a biscuit. Nearby were Kellogg's new screamin' fruit Squidgers, a f crushed fruit snack. Ninety per cent fruit and fruit juice - not bad. "Fruit Winders are just as good, but I haven't seen the Fruit Winder advert lately, they only do one for Screamers," they said knowledgeably. "But it's all scrummy stuff here - Fruit Shoots, Panda."

In the cereal section, even the packed-lunch-resistant parent is vulnerable. Every packet is emblazoned with temptation. There's a scene in Tony Parson's novel Man and Boy where the child is puzzled when his newly single dad offers him the sugary cereal, the one saved for holidays, instead of the no-fun everyday breakfast his mum usually gives him. It rings bells with every parent who has had to negotiate a deal: the sweetest cereals are allowed for "special" occasions, like Saturdays. On our research safari, nobody bothers with the Shredded Wheat or Weetabix, it's straight to "Weetos, yeah!" These are 36 per cent sugar, and the kids want the "toon popper" (plastic toy) inside. And it's "Frosties Chocolate, yeah" as well. That's 41 per cent sugar. But the one they really, really want is Nestlé's Cookie Crisp (40.8g of sugar per 100g). Here's why: "There's this guy, right. He has bouncy things on his shoes and jumps to a block of flats and there's these children eating Cookie Crisps and he says, 'Can I have some of yours?' Everyone puts their bowls out the window and it rains Cookie Crisps down from the sky into the kids' bowls."

Academics, consumer campaigners and health organisations are in no doubt that adverts contribute to the obesity epidemic. Obesity rates in children have doubled in the past 20 years. One in six children aged six to 15 is obese, as are 9 per cent of two to four years olds. Successive reports (from, to pick a few, the International Obesity Task Force, National Audit Office, Food Standards Agency, and Royal College of Paediatricians) in the past few months have reiterated the scale of the problem. Sitting tight and doing very little is not an option - that's partly what made us fat in the first place. Children should take more exercise, parents should exercise more control, all sides agree. But something else is keeping everyone on their toes: debating whether advertising junk food to children should be banned.

"Children are being targeted systematically," says Neville Rigby, director of policy and public affairs at the International Obesity Task Force. He makes comparisons between the junk food business and the tobacco industry of the past: "Unless we address the diet side of it no amount of jogging will compensate for the junk food." The proportion of fat in the average UK diet has increased. It used to account for 20 per cent of the calories we eat. Now that's up to nearly 40 per cent.

These organisations want children steered away from the foods and way of life that can make them overweight, develop diabetes and die early as adults. Banning adverts for unhealthy food aimed at children is one of their priorities. Not surprisingly, advertisers and food manufacturers beg to differ. The British Retail Consortium which represents the supermarkets and other shops, insists "an attack on food advertising misses the real cause of childhood obesity". Promoting these foods isn't what causes obesity, goes the advertisers and food manufacturers' line. Education, family environment and lifestyle matter more.

The Department of Health is investigating the link between food promotion to children and obesity. Ofcom is reviewing the codes which govern food advertising to children and should report by the middle of the year. Last month the Food Standards Agency, guardian of the nation's food safety, threw its hat into the ring with an action plan for improving children's diets, partly by changing the ways food is promoted to them.

This issue is hotter than the fattiest, saltiest, deep-fried slice of potato. In one corner is Scooby Doo, Thomas the Tank Engine and a host of other characters from children's TV, licensed to appear on children's foods, along with food industry giants and advertising magicians. On the other are, well, almost everyone else you'd expect to care about your health. Sustain, an alliance of health and consumer campaigns, led the most recent appeal to the Government. Another hundred groups (among them the British Dental Association, British Heart Foundation, Centre for Food Policy, Child Growth Foundation, Child Poverty Action Group, Children's Society, Co-operative Group, Consumers' Association, Food Commission, Guild of Food Writers, National Children's Bureau, National Union of Teachers and Unison) backed it and joined a worldwide appeal to the World Health Organisation to incorporate the marketing ban into its global anti-obesity initiative.

And then, in between the two camps, we have Tessa Jowell, Secretary of State for Culture, Media and Sport, who recently made clear she wouldn't give in to demands for a ban on the ads that make unhealthy children's food seem so tempting.

After sifting through all available research on the subject, last autumn's Food Standards Agency review, titled "Does Food Promotion Influence Children?", came to the conclusion that advertising does affect what children eat. Entirely predictable? Not if you believe the advertising and food and drink industries. "They don't see themselves as part of the problem," says Debra Shipley, Labour MP for Stourbridge, who has campaigned in Parliament on this issue. "It's not just that these products exist, it's that the manufacturers are doing everything within their power to make children want them - and persuade their parents to buy them. We have got to stop brainwashing tiny children, on TV and in schools with vending machines and sponsorship activities."

For once, instead of clearing up the remains of their nutritious tea, I watched TV with the kids. In under an hour, we saw commercials for Frosties, McDonald's Happy Meals, Hubba Bubbba 5 chewing gum and Kinder Surprise. The cartoon was sponsored by Coco Pops Crunchers. Research by the Food Commission, a watchdog group, showed that of all the cereals advertised in children's TV viewing time, 89 per cent were for high-sugar children's cereals. The advertising and food-manufacturing lobby claims that without ads for children, the choice and quality of their TV programmes and channels would reduce. How impoverished their lives would be.

The sums spent promoting confectionary are gobsmacking. Last year, £25m was spent on food advertising aimed at children and shown on the channels, and at the times, they watch. On terrestrial TV, there might be up to 10 adverts an hour for food high in fat, sugar and salt. Reading the Grocer, a trade magazine, for tantalising snippets about what advertisers are up to, I learn that the new Cadbury's Giant Fingers (20g each) hope to capture a significant slice of the lunchbox market and take on KitKat. The TV ads start in April, as part of a £4m package, including sponsorship of the Channel 5 Family Movie on Saturdays.

With less than half our children eating school dinners, lunchboxes are important, as are the foods targeted at them. These include processed products with more sodium than a child's daily allowance (Dairylea Dunkers Salt 'n' Vinegar Twist have 0.9g of sodium per 100g - remember: 0.5g is considered a lot) and sugary cereal bars. A survey which prized open the plastic lids of primary school children's lunchboxes found that 71 per cent included a bag of crisps, 60 per cent a biscuit or chocolate bar, and less than half had a piece of fruit. Nine out of 10 contained food too high in saturated fat, salt and sugar. Many children are eating twice the recommended amount of sugar. Add in breakfast cereals and their sugar intake soars.

And what of the children (like mine) whose entreaties have been stonewalled, despite their pleas that everyone else at school has blue Slush Puppies after swimming? Do they feel hard done by? Take a guess. We can't manipulate what they want to eat but - at home, at least - we can determine what our children do eat. Which is why we shop alone, if we can.

The standard defence of advertising is that it introduces consumers to new foods. There are 30,000 food products out there, says voice of the industry, the Food and Drink Federation. How dull it would be without them all. We'd be depriving children, taking their sweets away from them. The young cannot be sheltered from marketing and advertising, the most visible and entertaining aspects of commercial activity. "Children enjoy and remember advertisements, but this does not necessarily mean that they have an impact on their behaviour," says its defiant manifesto. "Food advertising does not dictate overall diet and health," states the Food Advertising Unit, set up by the Advertising Authority, an industry body.

And then there's Dr Jason Halford, a bio-psychologist and expert in "human ingestive behaviour" at the University of Liverpool, who says commercials aimed at children may actually stimulate them eat to more. He showed children - both the obese and lean - food adverts, and then made the same foods available immediately afterwards. Many of them - irrespective of their size - ate more than they did when they hadn't been shown the adverts. But as Dr Halford, points out, in most homes the high-fat and sugary foods like crisps and biscuits are as easy to get hold of as the healthier snacks, and will seem more attractive to appetites just whetted by TV ads.

It'll take a lot of investment and imagination to redress the balance. Who will put a persuasive case for a crunchy carrot and unsalted rice cake as the first thing a child wants to nibble after school? Tessa Jowell has urged advertisers to do just that. But the spokeswoman for the Food and Drink Federation explains that "advertising is used for brands to differentiate themselves in a competitive marketplace." Any volunteers for some creative advocacy for broccoli? "It would need to be a branded cabbage to be promoted," she admits. And what the industry does is add value - produce food they can charge more for. Of course, there wouldn't be adverts if the food wasn't invented. Oh, but then there's the argument that the food industry's role is to provide choice for consumers. So, what'll it be for lunch then? Dairylea Dunkers Jumbo Munch or an Attack-a-Snak Cheestring Chicken Wrap?