It is the world's biggest online business. But with questions being asked about its treatment of employees, what is it like to work at Amazon? Carole Cadwalladr lands a job in one of its giant warehouses and discovers the human cost of our lust for consumer goods

The first item I see in Amazon's Swansea warehouse is a package of dog nappies. The second is a massive pink plastic dildo. The warehouse is 800,000 square feet, or, in what is Amazon's standard unit of measurement, the size of 11 football pitches (its Dunfermline warehouse, the UK's largest, is 14 football pitches). It is a quarter of a mile from end to end. There is space, it turns out, for an awful lot of crap.

But then there are more than 100m items on its UK website: if you can possibly imagine it, Amazon sells it. And if you can't possibly imagine it, well, Amazon sells it too. To spend 10½ hours a day picking items off the shelves is to contemplate the darkest recesses of our consumerist desires, the wilder reaches of stuff, the things that money can buy: a One Direction charm bracelet, a dog onesie, a cat scratching post designed to look like a DJ's record deck, a banana slicer, a fake twig. I work mostly in the outsize "non-conveyable" section, the home of diabetic dog food, and bio-organic vegetarian dog food, and obese dog food; of 52in TVs, and six-packs of water shipped in from Fiji, and oversized sex toys – the 18in double dong (regular-sized sex toys are shelved in the sortables section).

On my second day, the manager tells us that we alone have picked and packed 155,000 items in the past 24 hours. Tomorrow, 2 December – the busiest online shopping day of the year – that figure will be closer to 450,000. And this is just one of eight warehouses across the country. Amazon took 3.5m orders on a single day last year. Christmas is its Vietnam – a test of its corporate mettle and the kind of challenge that would make even the most experienced distribution supply manager break down and weep. In the past two weeks, it has taken on an extra 15,000 agency staff in Britain. And it expects to double the number of warehouses in Britain in the next three years. It expects to continue the growth that has made it one of the most powerful multinationals on the planet.

Right now, in Swansea, four shifts will be working at least a 50-hour week, hand-picking and packing each item, or, as the Daily Mail put it in an article a few weeks ago, being "Amazon's elves" in the "21st-century Santa's grotto".

If Santa had a track record in paying his temporary elves the minimum wage while pushing them to the limits of the EU working time directive, and sacking them if they take three sick breaks in any three-month period, this would be an apt comparison. It is probably reasonable to assume that tax avoidance is not "constitutionally" a part of the Santa business model as Brad Stone, the author of a new book on Amazon, The Everything Store: Jeff Bezos and the Age of Amazon, tells me it is in Amazon's case. Neither does Santa attempt to bully his competitors, as Mark Constantine, the founder of Lush cosmetics, who last week took Amazon to the high court, accuses it of doing. Santa was not called before the Commons public accounts committee and called "immoral" by MPs.

For a week, I was an Amazon elf: a temporary worker who got a job through a Swansea employment agency – though it turned out I wasn't the only journalist who happened upon this idea. Last Monday, BBC's Panorama aired a programme that featured secret filming from inside the same warehouse. I wonder for a moment if we have committed the ultimate media absurdity and the show's undercover reporter, Adam Littler, has secretly filmed me while I was secretly interviewing him. He didn't, but it's not a coincidence that the heat is on the world's most successful online business. Because Amazon is the future of shopping; being an Amazon "associate" in an Amazon "fulfilment centre" – take that for doublespeak, Mr Orwell – is the future of work; and Amazon's payment of minimal tax in any jurisdiction is the future of global business. A future in which multinational corporations wield more power than governments.

But then who hasn't absent-mindedly clicked at something in an idle moment at work, or while watching telly in your pyjamas, and, in what's a small miracle of modern life, received a familiar brown cardboard package dropping on to your doormat a day later. Amazon is successful for a reason. It is brilliant at what it does. "It solved these huge challenges," says Brad Stone. "It mastered the chaos of storing tens of millions of products and figuring out how to get them to people, on time, without fail, and no one else has come even close." We didn't just pick and pack more than 155,000 items on my first day. We picked and packed the right items and sent them to the right customers. "We didn't miss a single order," our section manager tells us with proper pride.

At the end of my first day, I log into my Amazon account. I'd left my mum's house outside Cardiff at 6.45am and got in at 7.30pm and I want some Compeed blister plasters for my toes and I can't do it before work and I can't do it after work. My finger hovers over the "add to basket" option but, instead, I look at my Amazon history. I made my first purchase, The Rough Guide to Italy, in February 2000 and remember that I'd bought it for an article I wrote on booking a holiday on the internet. It's so quaint reading it now. It's from the age before broadband (I itemise my phone bill for the day and it cost me £25.10), when Google was in its infancy. It's littered with the names of defunct websites (remember Sir Bob Geldof's deckchair.com, anyone?). It was a frustrating task and of pretty much everything I ordered, only the book turned up on time, as requested.

But then it's a phenomenal operation. And to work in – and I find it hard to type these words without suffering irony seizure – a "fulfilment centre" is to be a tiny cog in a massive global distribution machine. It's an industrialised process, on a truly massive scale, made possible by new technology. The place might look like it's been stocked at 2am by a drunk shelf-filler: a typical shelf might have a set of razor blades, a packet of condoms and a My Little Pony DVD. And yet everything is systemised, because it has to be. It's what makes it all the more unlikely that at the heart of the operation, shuffling items from stowing to picking to packing to shipping, are those flesh-shaped, not-always-reliable, prone-to-malfunctioning things we know as people.

It's here, where actual people rub up against the business demands of one of the most sophisticated technology companies on the planet, that things get messy. It's a system that includes unsystemisable things like hopes and fears and plans for the future and children and lives. And in places of high unemployment and low economic opportunities, places where Amazon deliberately sites its distribution centres – it received £8.8m in grants from the Welsh government for bringing the warehouse here – despair leaks around the edges. At the interview – a form-filling, drug- and alcohol-testing, general-checking-you-can-read session at a local employment agency – we're shown a video. The process is explained and a selection of people are interviewed. "Like you, I started as an agency worker over Christmas," says one man in it. "But I quickly got a permanent job and then promoted and now, two years later, I'm an area manager."

Amazon will be taking people on permanently after Christmas, we're told, and if you work hard, you can be one of them. In the Swansea/Neath/Port Talbot area, an area still suffering the body blows of Britain's post-industrial decline, these are powerful words, though it all starts to unravel pretty quickly. There are four agencies who have supplied staff to the warehouse, and their reps work from desks on the warehouse floor. Walking from one training session to another, I ask one of them how many permanent employees work in the warehouse but he mishears me and answers another question entirely: "Well, obviously not everyone will be taken on. Just look at the numbers. To be honest, the agencies have to say that just to get people through the door."

It does that. It's what the majority of people in my induction group are after. I train with Pete – not his real name – who has been unemployed for the past three years. Before that, he was a care worker. He lives at the top of the Rhondda Valley, and his partner, Susan (not her real name either), an unemployed IT repair technician, has also just started. It took them more than an hour to get to work. "We had to get the kids up at five," he says. After a 10½-hour shift, and about another hour's drive back, before picking up the children from his parents, they got home at 9pm. The next day, they did the same, except Susan twisted her ankle on the first shift. She phones in but she will receive a "point". If she receives three points, she will be "released", which is how you get sacked in modern corporatese.

The Observer's Carole Cadwalladr outside the warehouse where she worked for a week. Photograph: Stephen Shepherd for the Observer

And then there's "Les", who is one of our trainers. He has a special, coloured lanyard that shows he's an Amazon "ambassador", and another that says he's a first aider. He's worked at the warehouse for more than a year and over the course of the week I see him, speeding across the floor, going at least twice the rate I'm managing. He's in his 60s and tells me how he lost two stone in the first two months he worked there from all the walking. We were told when we applied for the jobs that we may walk up to 15 miles a shift. He'd been a senior manager in the same firm for 32 years before he was made redundant and landed up here. How long was it before you got a permanent job, I ask him. "I haven't," he says, and he holds up his green ID badge. Permanent employees have blue ones, a better hourly rate, and after two years share options, and there is a subtle apartheid at work.

"They dangle those blue badges in front of you," says Bill Woolcock, an ex-employee at Amazon's fulfilment centre in Rugeley, Staffordshire. "If you have a blue badge you have better wages, proper rights. You can be working alongside someone in the same job, but they're stable and you're just cannon fodder. I worked there from September 2011 to February 2012 and on Christmas Eve an agency rep with a clipboard stood by the exit and said: 'You're back after Christmas. And you're back. And you're not. You're not.' It was just brutal. It reminded me of stories about the great depression, where men would stand at the factory gate in the hope of being selected for a few days' labour. You just feel you have no personal value at all."

Why haven't they given you a proper job, I ask Les, and he shrugs his head but elsewhere people mutter: it's friends of the managers who get the jobs. It's HR picking names at random. It's some sort of black magic nobody understands. Walking off shift in a great wave of orange high-vis vests, I chat to another man in his 60s. He'd been working in the Unity mine, near Neath, he told me, until a month ago, the second time he'd been laid off in two years. He'd worked at Amazon last Christmas too. "And they just let me go straight after, no warning or anything. And I couldn't have worked any harder! I worked my socks off!"

When I put the question to Amazon, it responded: "A small number of seasonal associates have been with us for an extended period of time and we are keen to retain those individuals in order that we can provide them with a permanent role when one becomes available. We were able to create 2,300 full-time permanent positions for seasonal associates in 2013 by taking advantage of Christmas seasonality to find great permanent employees but, unfortunately, we simply cannot retain 15,000 seasonal employees."

And this is what Amazon says about its policy relating to sickness: "Amazon is a company in growth and we offer a high level of security for all our associates. Like many companies, we employ a system to record employee attendance. We consider and review all personal circumstances in relation to any attendance issues and we would not dismiss anyone for being ill. The current systems used to record employee attendance is fair and predictable and has resulted in dismissals of 11 permanent employees out of a workforce of over 5,000 permanent employees in 2013."

It's worth noting that agency workers are not Amazon employees.

There's no doubt that it is hard, physical work. The Panorama documentary majored on the miles that Adam walked, the blisters he suffered, the ridiculous targets, and the fact that you're monitored by an Orwellian handset every second of every shift. As an agency worker, you're paid 19p an hour over the minimum wage – £6.50 – and the shifts are 10½ hours long. But lots of jobs involve hard, physical work. That's not the thing that bothers people. Almost everybody remains stoical in the face of physical discomfort and exhaustion. And they're Welsh: there's a warmth and friendliness from almost everyone who works there. My team leader is no corporate droid. He started on the shop floor, sounds like Richard Burton, and is gently encouraging. And yet.

"I've worked everywhere," a forklift truck driver tells me. "And this is the worst. They pay shit because they can. Because there's no other jobs out there. Trust me, I know, I tried. I was working for £12 an hour in my last job. I'm getting £8 an hour here. I worked for Sony before and they were strict but fair. It's the unfairness that gets you here."

An unfairness that has no outlet. In the wake of the BBC documentary, Hywel Francis, the MP for Aberavon, managed to get a meeting last week with Amazon's director of public policy, a meeting he's been trying to get for years. He's reluctant to speak about the complaints he's heard from his constituents but says that "the plant is exceptional in the local area in having no union representation. It's been a long haul to even get in there and find out what is going on." It's been a black hole where the lack of any checks upon its power has left a sense that everything is pared to the absolute bone – from the cheapest of the cheap plastic safety boots, which most long-term employees seem to spend their own money replacing with something they can walk in, to the sack-you-if-you're-sick policy, to the 15-minute break that starts wherever you happen to be in the warehouse. On my third morning, at my lowest point, when my energy has run out and my spirits are low, it takes me six minutes to walk to the airport-style scanners, where I spend a minute being frisked. I queue a minute for the loos, get a banana out of my locker, sit down for 30 seconds, and then I get up and walk the six minutes back to my station.

To work at Amazon is to spend your days at the coalface of consumerism. To witness our lust for stuff. This year's stuff includes great piles of Xboxes and Kindles and this season's Jamie Oliver cookbook, Save With Jamie (you want to save with Jamie? Don't buy his sodding book), and Paul Hollywood's Pies & Puds, and Rick Stein's India.

The celebrity chef cookbooks incense me. They don't even bother taking them out of the boxes. They lie in great EU butter mountain-sized piles at the ends of the aisle. Cook an egg on the telly and it's like being given a licence to print money for all eternity. The vast majority of people working in the warehouse are white, Welsh, working class, but I train with a man who's not called Sammy, and who isn't an asylum seeker from Sudan, but another country, and I spend an afternoon explaining to him what the scanner means when it tells him to look for a Good Boy Luxury Dog Stocking or a Gastric Mind Band hypnosis CD.

It's the Barbie Doll girl's Christmas advent calendar, however, that nearly breaks me. I traipse back and forth to section F, where I slice open a box, take another Barbie advent calendar, unpick the box and put it on the recycling pile, put the calendar, which has been shipped from China, passed from the container port to a third-party distributor and from there to the Amazon warehouse, on to my trolley and pass it to the packers, where it will be repackaged in a different box and finally reach its ultimate destination: the joy in a small child's heart. Because nothing captures the magic of Christmas more than a picture of a pneumatic blonde carrying multiple shopping bags. You can't put a price on that (£9.23 with free delivery).

Amazon's arrival has coincided with the decline of the high street in nearby town Briton Ferry. Photograph: Stephen Shepherd for the Observer

We want cheap stuff. And we want to order it from our armchairs. And we want it to be delivered to our doors. And it's Amazon that has worked out how to do this. Over time, like a hardened drug user, my Amazon habit has increased. In 2002, I ordered my first non-book item, a This Life series 1 video; in 2005, my first non-Amazon product, a secondhand copy of a biography of Patricia Highsmith; and in 2008, I started doing the online equivalent of injecting intravenously, when I bought a TV on the site. "We are the most customer-centric company on earth," we're told in our induction briefing, shortly before it's explained that if we're late we'll get half a point, and after three of them we're out. What constitutes late, I ask. "A minute," I'm told.

I grew up in South Wales and saw first-hand how the 1980s recession slashed a brutal gash through everything, including my own extended family. I've always known that there's only a tissue-thin piece of luck between very different sorts of lives. But then my grandfather worked in a warehouse in Swansea. In my case, there really is only a tissue-thin piece of luck between me and an Amazon life. I have a lot of time to think about this during my 10½-hour day.

At the Neath working men's club down the road, one of the staff tells me that Amazon is "the employer of last resort". It's where you get a job if you can't get a job anywhere else. And it's this that's so heartbreaking. What did you do before, I ask people. And they say they're builders, hospitality managers, marketing graduates, IT technicians, carpenters, electricians. They owned their own businesses, and they were made redundant. Or the business went bust. Or they had a stroke. Or their contract ended. They are people who had skilled jobs, or professional jobs, or just better-paying jobs. And now they work for Amazon, earning the minimum wage, and most of them are grateful to have that.

Amazon isn't responsible for the wider economy, but it's the wider economy that makes the Amazon model so chilling. It's not just the nicey nice jobs that are becoming endangered, such as working in a bookshop, as Hugh Grant did in Notting Hill, or a record store, as the hero did in Nick Hornby's High Fidelity, or the jobs that have gone at Borders and Woolworths and Jessops and HMV, it's pretty much everything else too. Next in line is everything: working in the shoe department at John Lewis, or behind the tills at Tesco, or doing their HR, or auditing their accounts, or building their websites, or writing their corporate magazines. Swansea's shopping centre down the road is already a planning disaster; a wasteland of charity shops and what Sarah Rees of Cover to Cover bookshop calls "a second-rate Debenhams and a third-rate Marks and Spencer".

"People know about their employment practices, and all the delivery men hate them, but do people remember that when they click? Probably not. We try and kill them with kindness," she says. "You can't put the genie back in the bottle." But then there is nothing else to try and kill them with. It's cheaper, often for her, to order books on Amazon than through her distributor. "We're upfront about it and tell people, but there is just no way to compete with them on price."

There is no end to Amazon's appetite. "It's expanding in every conceivable direction," Brad Stone tells me. "It's why I called my book The Everything Store. Their ambition is to sell everything. They already have their digital services and their enterprise services. They've just started selling art. Apparel is still very immature and is set for expansion. Groceries are the next big thing. They're going very strongly after that because it will cut down costs elsewhere. If they can start running their own trucks in major metro areas, they can cut down the costs of third-party shippers."

In the UK, I point out, everyone already delivers groceries: Tesco, Asda, Waitrose, Sainsbury's. "I suspect they'll acquire," he says. And everywhere it kills jobs. Shops employ 47 people for every $10m in sales, according to research done by a company called ILSR. Amazon employs only 14 people per $10m of revenue. In Britain, it turned over £4.2bn last year, which is a net loss of 23,000 jobs. And even the remaining jobs, the hard, badly paid jobs in Amazon's warehouses, are hardly future-proof. Amazon has just bought an automated sorting system called Kiva for $775m. How many retail jobs, of any description, will there be left in 10 years' time?

Our lust for cheap, discounted goods delivered to our doors promptly and efficiently has a price. We just haven't worked out what it is yet.

It's taxes, of course, that pay for the roads on which Amazon's delivery trucks drive, and the schools in which its employees are educated, and the hospitals in which their babies are born and their arteries are patched up, and in which, one day, they may be nursed in their dying days. Taxes that all its workers pay, and that, it emerged in 2012, it tends not to pay. On UK sales of £4.2bn in 2012, it paid £3.2m in corporation tax. In 2006, it transferred its UK business to Luxembourg and reclassified its UK operation as simply "order fulfilment" business. The Luxembourg office employs 380 people. The UK operation employs 21,000. You do the math.

Brad Stone tells me that tax avoidance is built into the company's DNA. From the very beginning it has been "constitutionally oriented to securing every possible advantage for its customers, setting the lowest possible prices, taking advantage of every known tax loophole or creating new ones". It's something that Mark Constantine, the co-founder of Lush cosmetics, has spent time thinking about. He refuses to sell through Amazon, but it didn't stop Amazon using the Lush name to direct buyers to its site, where it suggested alternative products they might like.

"It's a way of bullying businesses to use their services. And we refused. We've been in the high court this week to sue them for breach of trademark. It's cost us half a million pounds so far to defend our business. Most companies just can't afford that. But we've done it because it's a matter of principle. They keep on forcing your hand and yet they don't have a viable business model. The only way they can afford to run it is by not paying tax. If they had to behave in a more conventional way, they would struggle.

"It's a form of piracy capitalism. They rush into people's countries, they take the money out, and they dump it in some port of convenience. That's not a business in any traditional sense. It's an ugly return to a form of exploitative capitalism that we had a century ago and we decided as a society to move on from."

In Swansea I chat to someone whose name is not Martin for a while. It's Saturday, the sun is shining and the warehouse has gone quiet. We've been told to stop picking. The orders have been turned off like a tap. "It's the weather," he says. "When it rains, it can suddenly go mental." We clear away boxes and the tax issue comes up. "There was a lot of anger here," he says. "People were very bitter about it. But I'd always say to them: 'If someone told you that you could pay less tax, do you honestly think you would volunteer to pay more?'" He's right. And the people who were angry were also right. It's an unignorable fact of modern life that, as Stuart Roper of Manchester Business School tells me, "some of these big brands are more powerful than governments. They're wealthier. If they were countries, they would be pretty large economies. They're multinational and the global financial situation allows them to ship money all over the world. And the government is so desperate for jobs that it has given away large elements of control."

It's a mirror image of what is happening on the shop floor. Just as Amazon has eroded 200 years' worth of workers' rights through its use of agencies and rendered a large swath of its workers powerless, so it has pulled off the same trick with corporate responsibility. MPs like to slag off Amazon and Starbucks and Google for not paying their taxes but they've yet to actually create the legislation that would compel them to do so.

"They are taking these massive subsidies from the state and they are not paying back," says Martin Smith of the GMB union. "Their argument is that they are creating jobs but what they are doing is displacing and replacing other jobs. Better jobs. And high street shops tend to pay their taxes. There is a £120bn tax gap that is only possible because the government pay tax benefits to enable people to survive. When companies pay the minimum wage they are in effect being subsidised by the taxpayer."

Back in Swansea, on the last break of my last day, I sit and chat with Pete and Susan from the Rhondda and Sammy, the asylum seeker from Sudan. Susan still wants a permanent job but is looking more doubtful about it happening. Her ankle is still swollen. Her pick rate has been low. We've been told that next week, the hours will increase by an hour a day and there will be an extra day of compulsory overtime. It will mean getting their children up by 4.30am and Pete is worried about finding a baby-sitter at three days' notice. When I ask Sammy how the job compares with the one he had in Sudan, where he was a foreman in a factory, he thinks for a minute then shrugs: "It's the same."

There have always been rubbish jobs. Ian Brinkley, the director of the Work Foundation, calls Amazon's employment practices "old wine in new bottles". Restaurants and kebab shops have done the same sort of thing for years. But Amazon is not a kebab shop. It's the future. Which may or may not be something to think about as you click "add to basket".