I have a secret addiction. Often it's one packet a day, sometimes it's more. Occasionally I'll go cold turkey. But whenever I'm depressed or desperate or drunk I'll have a craving. I know all the good places to score. Street corners are best. The relief is immediate - there's a rush, especially in the first few moments. But afterwards I feel guilty. I'll bury the evidence in my bag before I reach home. My name is Louise and I eat crisps. I've given up cigarettes and weaned myself off chocolate. But any crisp will do. I'm not alone. If all the empty packets we buy annually in Britain were put end to end they'd encircle the planet. Fifty times. That's 12 billion packets a year. We buy more bags of crisps than the rest of Europe put together, second only to the United States. At Christmas, sales rise by 20 per cent.

As an adult I know only too well that I'm eating a fatty, salty junk food. But imagine what crisps do to children, their most ardent fans. More than two-thirds of school children have a packet of crisps in their lunch box every day. My habit began at the age of three when a party meant fizzy pop, chocolate cake and bowls piled high with crisps. I remember smoky bacon-flavoured twists that were deliciously moreish. Salt and vinegar chipsticks that stuck to my teeth. Buttery fluorescent tubes that turned my fingertips orange.

What I didn't know then was that they were shaping my diet for years to come. Scientists have discovered that our taste buds memorise the flavours we eat when we're young. Munch crisps between six or 12 times when we're children and we literally develop a taste for them. Back when I bought a packet of crisps on the way to school with my lunch money I was nurturing a love for salt (one packet is three times as salty as sea water and contains half the recommended daily salt intake for a six-year-old). Combine the salt with fat (half the fat content is the unhealthy saturated kind) and artificial flavouring (big brand crisps all contain monosodium glutamate among other enhancers) and it's easy to see why they have become my crack cocaine.

The first sensation when I bite into a crisp is technically known as 'mouth feel'. This is the heady, pleasurable mixture of texture and flavourings. The 100 specialised receptor cells on each taste bud are mainlined to pick up five different flavours: sweet, sour, salt, bitter and umami (which is triggered by flavour enhancers). These molecular launching pads on the tongue respond to the flavour chemicals and initiate a chemical reaction. The reaction sends signals along nerve fibres to a taste centre in the brain. A crisp's flavours - they're salty, sweet and further enhanced by monosodium glutamate - mean my brain instantly thinks it's party time.

According to Joanna Blythman, author of Shopped: the Shocking Power of British Supermarkets (Fourth Estate): 'Crisps have a particularly pernicious effect on children. They warp their sense of taste. I see children walking to school, eating them for breakfast, eating them for lunch and at the bus stop. Sometimes three packets a day. Frankly, I think giving them to kids as a "food" counts as neglect.'

The irony, she says, is that as recently as the Eighties, crisps were considered to be healthy. 'The thinking went that at least they were savoury not sweet,' she says. 'People thought it was OK to eat them.'

Invented 151 years ago in Saratoga, North America, a disgruntled chef made them by mistake when a diner complained that his potato chips weren't thin enough - crisps became the epitome of food on the run while I was growing up. Made from potatoes and oil - two of the most basic, cheapest ingredients to be found - they symbolised popular, predictable, easy food that everyone felt safe with. They suited late 20th-century sociological trends in Britain: more commuters, fewer people going home for lunch. Everyone ate them, whether it was the six-year-old at a birthday party, the office worker stuck at his desk, the beer drinker lingering in the pub, the hostess with a drinks party to organise and an aversion to fiddly canapes.

Not any longer. In the past year the levels of obesity - especially among children - has caused increasing alarm, and crisps are seen as one of the main culprits. In October, manufacturers were hauled up in front of a select committee for health. Three weeks ago, the government announced proposals to ban junk-food commercials until 9pm and that foods will be labelled in a traffic-light warning scheme where crisps will be 'red' and vegetables 'green'. Meanwhile, the Food Standards Agency (FSA) has launched a campaign warning against salt, and the pressure group Sustain is lobbying for a children's food bill and recommending an end to crisps being stocked in school vending machines.

Crisp manufacturers face the kind of criticism better known by the tobacco industry. In America, lawyers are already working on junk-food litigation cases. None has succeeded so far, but the day may come. Crisp makers are faced with a problem: how to sell more packets but keep the parents and politicians at bay.

Martin Glenn, president of PepsiCo UK which owns Walkers, is upfront: he eats two packets of cheese and onion crisps a day. 'What's the point of having a sandwich without crisps?' he says. It's easy for him to get hold of a bag - the Walkers base in Leicester is the biggest crisp factory in the world. It produces 14 million packets a day, which makes Glenn the most powerful executive in the industry.

I'm taken on a tour of the production line. From the 20-ton lorries piled high with pink Lady Rosetta potatoes through to the balletic robots packing up the boxes onto pallets, it takes 40 minutes to make a bag of crisps. The spuds, which are grown from their own seeds, are swooshed along tunnels by torrents of water, then whizzed around in rough-edged barrels where the skins are scrubbed away. They're mechanically sliced into transparent white circles and sprayed with more water to get rid of the starch. For three minutes they float in a bubbling vat of oil (the temperature of which they won't tell me. 'Trade secret,' says Simon Ely, the quality manager.)

Next they fall, like identical pale gold autumn leaves, into tubs of flavouring - or 'seasoning' as Simon prefers to call it. Finally, automated machines deftly pop the just-fried crisps into the iconic red, blue or green packets. Apart from one woman with the job of spotting any reject spuds, the potatoes are peeled, sliced, fried and boxed up without the touch of human hand.

As a crisp lover I imagined going onto the manufacturing floor would be like Charlie entering Willy Wonka's chocolate factory. Instead I'm curiously pleased to leave. The air is heavy with salty, cloying fat. It's so hot sweat trickles down my back. The sound of the machinery is disorienting. It's like being in a vast noisy chip shop. There's no doubting that it's an impressive operation: this factory never stops. It's a 24-hour, seven-days-a-week operation. But the noise, lights and smell are overpowering.

Martin Glenn has proved himself as a clever marketer - he's the man who made local Leicester boy and former England football captain Gary Lineker the public face of Walkers. He's expanded the brand - two years ago he spotted a gap in the market between premium crisps and the basic range. The result was Sensations, a mid-market line targeted at 18-24 year olds with flavours reminiscent of a drunken Friday night out at a restaurant - Thai Sweet Chilli, Four Cheese and Red Onion.

But now Glenn needs to win the obesity debate. Companies like Walkers are being forced to change, he says. 'A few years ago we would have taken a libertarian approach to health: if you want to abuse the product that's your problem. Not any longer.' However, he's wary of the traffic-light labelling system: 'too complicated'. And dead against the advertising curfew: 'It's been tried in Europe and doesn't work.'

His solution is Walkers' 'Britain on the Move' campaign. It's a sign of the sensitivity within the business that this latest commercial for Walkers doesn't include a single packet of crisps (reminiscent of Silk Cut's award-winning adverts in the Eighties which showed the brand's famous purple and white colours but no actual cigarettes). Instead the £3million advert features Madness' 'One Step Beyond' and Gary Lineker promoting 'walk-o-meters' - pedometers which measure how many steps you walk each day. Some 1.5 million have already been made. Crucially they're free - you don't have to buy lots of crisps to receive one of the branded little gadgets.

The pedometer campaign is smart yet simple: Walkers means walking (not slacker couch potatoes). 'People remember small messages,' explains Glenn. 'We're telling them to make changes in small steps - both literally and figuratively. The only people who solve weight problems are those who incorporate physical exercise.'

He uses the same language as the dieticians I speak to. Words like 'moderation' and 'balance' pepper the conversation as much as 'smoky' and 'bacon'. But isn't he having his crisp sandwich and eating it too - winning publicity but telling us all that if we're fat it's our fault? Glenn disagrees: 'When the obesity storm blows over - which I think it will - the legacy will be people being more conscious about exercise. When you have one of these devices they change your life. You walk up the stairs. You get off the Tube one stop earlier.'

His argument is that there is a 'snobbishness' behind the debate. Global brands have become an easy target and the middle classes are the only ones who are taking notice. 'You can't go anywhere without someone reading Fast Food Nation. Processed food now equals evil food. But the food puritans are talking to themselves and everyone else is tuning out. Food should be a pleasure and you don't have to be Nigella Lawson to take pleasure from it.

'We all want treat food in our diet. I know how many vegetables and pieces of fruit of fruit I need. I also know a packet of crisps is a small calorie hit [185 calories in a 34g packet]. That they taste fantastic, that they don't cost much, that they cheer me up.'

I'm sent away with a box of cheese and onion crisps under my arm that, after my factory tour, I'm not sure I have the appetite for. The pedometer promotion is a clever one but I wonder if they're a passing craze.

The label on a typical bag of cheese and onion flavoured crisps reads: potatoes, vegetable oil, cheese and onion flavour, flavour enhancer (monosodium glutamate, sodium 5'ribonucleotide), cheese powder, colour (annatto, paprika extract), partially hydrogynated soya bean oil, salt.

I ask The Observer 's health expert, Dr John Briffa (whom Martin Glenn describes as a 'sanctimonious food snob') why he hates the humble crisp. 'Where to start?' he says. 'First there's the potato. Not a particularly healthy food.' Then there's the salt. 'They say they're cutting back but that's like saying, "One or two drops of cyanide?"' Look closely at the label. It's actually difficult to work out the salt rating. It's usually listed as 'sodium'. In fact, you need to multiply the sodium content two-and-a-half times to get the accurate amount of salt. 'Saltiness is one of the things that keep people coming back to crisps,' says Briffa. It also, of course, makes us thirsty (useful for Walkers owner Pepsi).

Some manufacturers' flavours include aspartame (E-number 951), an artificial sweetener also used by drug companies to disguise bitter-tasting medication. The flavour enhancer monosodium glutamate (E621) has caused some controversy - critics say it can lead to complaints of migraines, asthma and heart palpitations.

Colourings such as tartrazine (E102), quinoline yellow (E104) and sunset yellow (E110) have been researched by the UK Asthma and Allergy Research Centre, which blames them for 'significant changes in children's hyperactive behaviour'. There is an ongoing debate between the manufacturers who argue that these ingredients are harmless and campaigners who disagree (aware that consumers are wary of E numbers, labels have reverted to the chemical names instead). The FSA tends to preach 'moderation' while pushing the line that it would be much better to eat more fruit and vegetables.

'But what's thoroughly bad about crisps is the trans fats, which they won't own up to on the labels,' says Briffa. Most people know the difference between healthy fat such as olive oil and artery-clogging saturated fats. Crisp manufacturers must list the percentage of saturated fat in each packet, but trans fats - so called 'killer fats' - have only recently been in the headlines and are not listed. They're the result of manufacturers using hydrogynated oil - oil that has been treated to extend a product's shelf life.

According to the FSA: 'Trans fats increase the risk of coronary heart disease because of the effect they have on blood cholesterol. Evidence suggests the effects of trans fats are worse than saturated fats.' The Ban Trans Fat campaign goes further. 'Trans fats make arteries rigid and cause diabetes. They're in food to increase shelf life but they decrease human life.' Twenty nine per cent of the fat content in an average packet of crisps is made up of trans fat. In America, all food manufacturers must list the amount of trans fats on labels by January 2006. There are no such moves in this country.

Three years ago, a group of Swedish researchers discovered another problem: acrylamide. This chemical is produced when foods rich in starch have been cooked to high temperatures. Scientists already knew of its existence in tobacco smoke but the study suggested that, following tests on rats, acrylamide was carcinogenic and could be harmful to humans. A year later, another study contradicted these results and the FSA said that there was nothing to worry about.

John Briffa puts crisps on his hit list below fizzy drinks and burgers. 'I wouldn't call crisps a food. Even chocolate has some nutritional value. Or at least it doesn't contain much bad stuff. Far better to eat a packet of nuts - they reduce the risk of heart disease, are less fattening than you imagine, and sate the appetite.'

Joanna Blythman - though willing to eat a packet of Pringles on a long-haul flight when she's 'desperate' - goes further. 'The manufacturers make out that they're a "meal experience". That they're "hand-held snacks". But it's not just the fact that they have no nutritional value. It's a psychological thing, too. We're not sitting down to eat and being nurtured. We're not taking time to appreciate food. Eating a packet of crisps devalues the whole experience. It says something pretty pathetic about our culture. We're short-changing ourselves.'

So where does the industry go from here? Listen to the critics and opening a family pack of chipsticks will be as socially unacceptable as lighting up at a baby shower.

Not if the manufacturers have their way. They're trying all kinds of tactics to win the health debate. Some are branching out into fried vegetable crisps (unfortunately these are as calorie heavy as the usual kind). Packets come with all kinds of chummy messages from the manufacturers making out that they're full of wholesome values. Kettle Foods boasts about its organic pedigree; next year, Walkers plans to start increasing the ratio of sunflower to palm oil. Sainsbury's, meanwhile, is launching two new lines - a 'salt your own' bag of crisps and a no-salt packet. Andrew McTeare, its crisp buyer and a one-packet-a-day man, is blunt about the commercial advantages: 'There's been a decline in sales recently due to dietary concerns. We decided to ramp up the health element. This is a real growth area for us.'

In fact, a small company in Kingsbridge, Devon, had the idea of no-salt crisps six years ago. Repeatedly when I researched this piece, if a nutritionist did admit to eating crisps they'd name Burts potato chips.

The company is run by Jonty White and Nick Hurst, an ex-city broker and estate agent. They support local growers with a fair trade deal on their potatoes and the only supermarket they'll sell into is Waitrose (90 per cent of the business is with independent delis which they won't allow Waitrose to undercut). Their 35-strong workforce make six million packets a year.

Burts comes with endorsements from Rick Stein, Antony Worrall Thompson, even Nigel Slater who describes them 'as thin and fragile as a dragonfly's wing, this is the nearest you will get to a traditional handmade game crisp'. Each packet tells you who has fried the contents and which field the potatoes come from. They've been making no-salt crisps for six years, but their latest range is a no-salt crisp for children which comes in a smaller 25g bag. Five per cent of the profits will go to Great Ormond Street Hospital where Jonty's son Archie, who is autistic, has received treatment. 'I've put my heart on the line with these crisps,' he says. 'And Archie too. These are about me and him. When you have a son who is autistic, who never gets to be the star, this is a big thing.' A Great Ormond Street nutritionist worked with them on the product and endorses it on the packet.

Of course there is no getting around the fact that they're making crisps, not carrots. They agree: 'We'd be the last to claim we're whiter than white,' they say. But they do score some health points. Each packet has fewer calories than the big brands. They only use good quality sunflower oil and no artificial additives. The no-salt flavour is surprisingly delicious and because there's no salt there isn't the compulsion to polish off the bag in 30 seconds (not least because at 65p a pack they're expensive).

Next time I fall off the wagon, I resolve to eat only no-salt crisps. John Briffa says this is 'like moving the deckchairs on the Titanic', but as they say in the addiction industry, one day at a time.

The crisp taste test

Low-fat crisps

Tester: Michelin-starred chef Philip Howard of the Square, London W1 (020 7495 7100).

Aldi low fat crisps: 'Thick, crunchy and almost natural tasting. Not bad considering they're low in fat.' 4/5
Walkers Lites: 'Taste saltier, but fine.' 3/5
Tesco reduced fat: 'Fine, but dull.' 3/5

Hand-cooked crisps

Tester: Michelin-starred chef Tom Aikens of Tom Aikens, London SW3 (020 7584 2003)

Tyrrells lighted sea salted: 'Great.' 4/5
Pipers Anglesey sea salt crisps: 'They taste really earthy.' 4/5
Burts lightly salted: 'These actually taste of potato.'3/5
Waitrose handmade crisps: 'Very thick but not bad.' 3/5
Kettle chips. 'Nice and crunchy.' 3/5

And the best for kids? Burts no-salt crisps for children (burtschips.com). Five per cent of the profits go to Great Ormond Street Hospital.