They're fried in fat and smothered in salt, but still we eat a heart-stopping 6bn packets of them a year. So why do we have an unhealthy obsession with potato crisps?

In an unremarkable suburb of Leicester called Beaumont Leys is a big factory – or actually two, side by side. But let's not split hairs already. The point is that it's big; a winding 10-minute march from reception round to the delivery bays.

These bays are where the raw material comes in, which is potatoes. The variety changes with the season, depending on whether they've come straight from the fields in summer, or from storage during winter. There are Hermes, Saturna and, right now, round, pink-cheeked Lady Rosetta.

Let's follow her, briefly. She is washed out of the truck – shedding any small stones or vestiges of earth she may be clinging to – and carried by stainless steel conveyor belt to a spinning drum, where she's peeled of her reddish skin.

She then passes across an inspection belt, where practised human eyes beneath faintly ridiculous but absolutely obligatory hygenic hair-nets hunt out hidden blemishes. Then, razor-sharp rotating blades slice her into 1.3mm slivers of starch and water.

Next, the excess starch is washed away (or the slivers will stick together), and the excess water dried off (it plays havoc with boiling oil), and hey, it's frying time: three brief but, one can only imagine, intense minutes in a 5,400-litre tank at 180C.

Out come the slices, all curling and golden and smelling (believe me) very good, whereupon a fiendishly smart automated scanning device gives them all the once-over once more, shedding those that look less than perfect. Next it is into the big drum for seasoning, which you're not allowed to see because it's top secret. Then weighing and bagging (more smart machines), and that's it: in less than 20 minutes, Lady Rosetta has become a packet of crisps.

This doesn't, though, give a true impression of the grandeur of the whole operation. This factory, belonging to Britain's largest crisp manufacturer, Walkers, is the biggest crisp factory in the world. It processes 800 tonnes of potatoes a day. It has six, 200m-long production lines, each of which turns out three tonnes of crisps an hour. That's maybe 120,000 small 25g packets. Per hour. Times six.

And this is only one of Walkers's seven UK crisp plants. Between them, they produce 10m packets a day, satisfying just under half this country's appetite for potato chips.

In short, we eat an awful lot of crisps. They are a national obsession. Practically everyone has a favourite flavour, or an unexpected craving, while even those who don't like them feel strongly – worrying, as chef Jamie Oliver has done very publicly, that this very British habit is doing untold damage to the health of the nation, particularly its children.

And when you consider we get through an estimated 6bn packets of crisps and 4.4bn bags of savoury snacks a year – around 150 packets a person – you do wonder what our love affair with crisps is doing to us. Looked at by tonnage, we consume more crisps, crackers and nuts than any other European country.

Unsurprisingly, though, the people at the Walkers factory wax positively lyrical. "There is," says James Stillman, head of research and development, "the physical experience. The crunch, the smell, the taste, how the salt dissolves on your tongue, how the flavours develop in your nose. Take our Sensations Thai Sweet Chilli: put one in your mouth and think. There's a five-second journey going on there, but you won't get it unless you really think."

It is, claims Stillman, nothing short of an emotional experience – "there's a great deal of anticipation in opening a packet of crisps" – and if so, it's an emotion that a great many of us share. Hardly anywhere else in the world, with the exception of America, do people consume fried potato slices in the manner, the variety and the quantity that we do, with well in excess of 100 varieties to choose from.

Elsewhere in Europe, the potato chip is a savoury something served with an aperitif (a complement, say, to the olive). In Britain, it's a food in its own right, or, as the Savoury Snacks Information Bureau puts it, "indisputably an integral part of the British culture".

In fact, muses food writer Matthew Fort, who confesses to a love affair with crisps dating back to the days of Smith's Salt 'n' Shake: "Crisps are our olives. The continentals once had plain olive oil. Now there's extra virgin, single estate, first cold pressed, extra virgin single varietal first cold pressed – you name it. We used to have plain ready salted; now there's any number of flavours, as well as traditionally cut, individually hand fried and the rest."

Hardly anywhere else is it possible to walk into a supermarket, corner shop, newsagent, petrol station or pub and expect to see arrayed before you a dozen or more brands, styles, varieties and flavours of crisps including (seriously) Balti Curry, Steak & Ale Pie, Chargrilled Chicken, Chilli con Carne, Jalapeno & Coriander, Taw Valley Cheddar & Caramelised Shallots, Spaghetti Bolognese, Aloo Masala, Xtra Spice Buffalo Wing, Argentinian Steak and BBQ Kangaroo.

But despite such esoteric offerings, Walkers's – and the UK's – top five has remained unchanged for years: in descending order, Cheese & Onion, Ready Salted, Salt & Vinegar, Prawn Cocktail and Chicken. "We're creatures of habit," says Stillman. "We like what we like, but we occasionally like to experiment. At any one time, Walkers will probably have 15 flavours in the market: the first five are generally the same, the other 10 will be changing pretty much constantly."

In continental Europe, by contrast, you're basically stuck with plain or, for some reason, paprika. We, though, are besotted: when Walkers ran its Do us a Flavour competition last year, in which the nation was invited to invent a new crisp flavour, it received 1.2m entries (the most memorable suggestion, Stillman says, was Ear Wax). This year, in parallel with the World Cup, the company ran its Flavour Cup, "a celebration of national cuisines from around the world". The popular winner was English Roast Beef & Yorkshire Pud, confirming that at heart, we see the crisp as something uniquely, quintessentially British.

Except, of course, it isn't. Or at least, it probably isn't. The crisp was allegedly born in Cary Moon's Lake Lodge (or Lake House) restaurant in Saratoga Springs, New York, on 24 August, 1853, when a former tracker called George Crum, son of a Native-American mother and an African-American father, got fed up with a customer (who may, or may not, have been rail magnate Cornelius Vanderbilt) sending back his fried potatoes because they were too thick for his liking.

The third (or, according to some accounts, fourth) time this happened, Crum, enraged, sliced the offending solanum tuberosum into wafer-thin slivers, deep-fried and over-salted the result, and sent the dish out again hoping the guy would choke on it. But Vanderbilt (if it was he) loved them – you can't go wrong, tastebud-wise, with starch, fat and salt – and Saratoga Chips became a staple of the restaurant's menu.

(I say "probably not British", incidentally, because a recipe for "fried potato shavings" was reportedly printed in America as early as 1832, in a book based on an even earlier collection of recipes from England. There again, when the first confirmed sighting of native British crisps was reported, in 1913, they were being made in London by a man called Carter, who had supposedly stumbled across them in France. So who knows?)

Anyway. In 1920, Smith's Potato Crisps Company Ltd was formed in Cricklewood, north London, with Mrs Smith peeling, slicing and frying the potatoes in the garage and Frank Smith packing them into greaseproof bags (later with a pinch of salt in a twist of blue paper inside) and selling them across London from his pony and trap. The firm was so successful it had moved to new premises and hired 12 full-time staff before its first year was out.

The company ran into trouble in the Depression, however, undergoing the humiliation of being rescued by its Australian subsidiary. But hard times proved the start of something big and beautiful for Mr Henry Walker, a successful pork butcher in Leicester. In the years immediately after the second world war he was facing bankruptcy, as rationing saw his shops in Cheapside and Oxford Street, London, cleared of meat before 10am, with nothing left to sell.

"It was a choice between ice-cream and crisps," former managing director Gerry Gerrard told the Leicester Mercury years later. He went for crisps because of the difficulties of handling meat and dairy products together. Walkers began in 1949 above the Oxford Street premises, with a staff of eight and Gerrard himself as head cook. The crisps were hand cut with a vegetable slicer, cooked in a chip-shop fryer, sprinkled with salt and sold for thruppence a packet under the slogan Potato Crisps by Walkers: Guaranteed Absolutely Pure.

They went down a bomb, and Walkers – which long ago swallowed Smith's, and is now part of the mammoth PepsiCo conglomerate – never looked back (helped in no small measure, since 1994, by the inspired choice of local lad Gary Lineker to front its advertising campaigns). We're no closer, though, to knowing why crisps are so big in Britain. What makes us, and so few others, so peculiarly partial to the potato chip?

There are plenty of theories. For Fort, it's mostly down to our unique relationship with the potato. "The potato has iconic status in this country; it's a subsistence food," he says. "A love of the potato is hard-wired into our gastronomic DNA. Plus, we've always been a grazing, snacking culture – look at our eating opportunities, we have more than anyone else: breakfast, elevenses, lunch, tea, high tea, supper, dinner . . . The French, the Italians, the Spaniards, eat twice a day, max. They're not snackers. The crisp is the perfect food for us."

Stillman reckons it has a lot to do with our high consumption of sandwiches, for which crisps are "an ideal complement", and of beer (ditto): "The creaminess of the potato, the salt and sweetness of the flavouring, the bitter of the beer; it all works." Felicity Lawrence, author of a brace of deeply scary books on the darker side of Big Food, also thinks pubs have something to do with it, but believes the underlying reason is that Britain industrialised earlier than most of the rest of Europe.

"Other countries maintained a more direct connection with their food and the land," she says. "We've been producing processed food for much longer, and consuming it too – there was a need for fast food from a very early stage, because people were working long hours in the factories." The crisp, then, is one of the earliest and most successful products of the long and happy marriage between industrialised food and a cheap, abundant crop.

Although the spectacularly competitive British market (remember Golden Wonder?) has been evolving and expanding pretty much since the crisp first arrived, aficionados point to two key revolutionary events: game-changing moments. The first was in the late 1950s, when years of kitchen experimentation by the late Joe "Spud" Murphy, proprietor of the cunningly named Tayto crisp company in Ireland, culminated in the invention of what is generally (though not, crisp history being a much-disputed field, universally) agreed to be the world's first crisp seasoning: Cheese & Onion.

The second major event was the arrival on these shores, in 1987, of an Oregon businessman called Cameron Earl, who brought with him a concept known as the Kettle Chip: thick, gnarled, irregular, crunchy, authentically flavoured and (naturally) more expensive. This was the premium product the hitherto classless world of the crisp had been waiting for, and it wasn't long before we saw an array of home-grown, artisan-inspired, hand-fried, organic rivals: Tyrrells, Burt's, Piper's and the rest. Walkers jumped in, too, with Sensations.

These are mostly known as "sharing" crisps, because they're sold in bigger bags, for more sociable consumption, and they're changing the shape of the market and the way we eat crisps. "They're for sitting on the sofa watching The X Factor," says Stillman, "not munching with your lunchtime sandwich." Sales of individual packets are falling slowly as sales of sharing packets, worth £370m last year, rise.

The British Heart Foundation's poster warning about the health risks of eating too many crisps. Photograph: PR handout

A posher product image, though, does not make for an inherently healthier product. Sharing crisps are almost all still cooked in fat and sprinkled – most of them – with salt (albeit Maldon sea salt) just as much as their down-to-earth cousins.

Four years ago, the British Heart Foundation famously warned that half of all British children were, in effect, drinking five litres of cooking oil a year by virtue of their packet-a-day habit (crisps are a staple in 69% of lunchboxes). More alarmingly, nearly a fifth of British children apparently eat two packets a day. Soaring rates of obesity and Type 2 diabetes were, the foundation warned, the consequences.

The crisp manufacturers complained of unfairness, inaccuracy and exaggeration, and the Savoury Snack Information Bureau – among other things, an active and effective rebuttal service founded to "ensure balanced reporting on the nutritional aspects of savoury snacks in the UK – swung into action. But the industry was stung, and has responded. "It's fair to say awareness has moved on," says Victoria Taylor, a senior dietician at the foundation.

"There have been reductions in salt content and sugar content and saturated fat intake, which is good, although crisps are still fried in fat, so calorie-wise that's not marvellous. There's no more advertising of junk food on children's television, although it's still on programmes lots of children watch. But we need to go further. It's all a question of balance. There are no individual foodstuffs I'd say you should never eat. But if you're eating something once or twice or more a day, then there's no room in your diet for the other foods you need."

In Leicester, they know the numbers off by heart: savoury snacks account for just 1% of saturated fat in the average UK diet, they say. Walkers has spent £20m in research and development since 2003 to make its crisps healthier. Most now contain up to 80% less saturated fat and 55% less salt than they did in 2006. New ranges such as Baked and SunBites contain between 30% and 70% less total fat, and 45% less saturated fat, than standard crisps.

"The point," says general manager Ian Ellington, "is that we have to make a product that consumers want. In the longer term, we're all moving towards consuming less fat and fewer calories, to making healthier choices. If we don't adapt and transform our portfolio, meet those needs while continuing to deliver taste and texture, then there won't be a Walkers brand."

Others are more sceptical. "It's just an idea of pleasure," says Lawrence. "Don't get me wrong, there's nothing wrong with enjoying a pack of crisps every now and again. But the truth is we shouldn't be eating them often; and that's the problem. Because they're selling so little, a packet of air and a few bits of something very cheap, the only way they can make money is by constantly reinventing themselves, and by making sure we eat an awful lot of them."

So, everyone. Are you more Cheese & Onion, or Taw Valley Cheddar & Caramelised Shallots? Personally, I can never decide.

• This article was amended on 1 September 2010. The original said the Britons consume more crisps, crackers and nuts than everyone else in Europe put together. This has been corrected. It also said that larger ("sharing") bags of crisps now account for 29% of the UK crisp market, against 25% five years ago. This has been deleted pending further checks on whether this holds true for the whole market, or specific companies only.