On a sunny morning in Birmingham city centre, Charlene Powell is standing guard over four carrier bags.

They are not all hers, she says hastily. She has come shopping with her mum, but between them they have done well out of their trip to the city’s new Primark megastore, a five-floor retail palace that was named by Guinness World Records as the biggest clothing store in the world when it was unveiled in April.

“I’m going on a hen night, so I’ve been trying to find some bits and pieces for that – there’s a unicorn theme on the Saturday night. And I’ve got some bits for my son because they sell all the Fortnite stuff in there. And some bits for my little girl for summer,” says the 30-year-old from Kidderminster. Charlene hasn’t shopped in Primark much before, but she is coming back; she can come by train and be back in time for the school pickup.

Judging by the number of branded brown paper bags swinging from arms or hooked over pushchair handles in Birmingham city centre, she is not alone. If the high street is dying, then Primark, which will mark 50 years in business in June, appears to be immortal. Sales rose by 4% in the six months to March, even as established brands, such as Debenhams and LK Bennett, were going to the wall. And it is remarkable that the biggest British fashion retailer in Britain by volume (if not by value) has done it all without venturing into online shopping. If you want Primark, you still have to get off the sofa and buy it – and millions happily oblige.

The Birmingham megastore is almost a tourist destination, what with its in-store blow-dry bar, beauty salon and Disney-themed cafe that is catnip to preschoolers. There have been coach trips from Scotland, with customers filming on their phones as they walk in, as if it were a theme park. But people are not just browsing; they are buying. Why not, when you can get a bikini for £6 or flip-flops for less than a takeaway coffee? These are pocket-money prices for grownups, and they are not just attracting shoppers on a tight budget.

Discount retailers have also discreetly helped the squeezed middle – people who are not exactly hard up, but have still felt the pinch over the past decade – ride out what would otherwise have been painful downward pressure on their living standards. Middle-class families have stealthily downshifted to Lidl or Ryanair, and started stocking up on cheap children’s T-shirts and leggings in “Primani, darling” (as the store was nicknamed when 00s fashionistas realised some of its lines could, from a distance, pass for designer). These days, Primark competes for customers with Marks & Spencer and Next, not just with H&M or New Look.

Facebook Twitter Pinterest Shoes on sale at Primark. Photograph: Universal Images Group North America LLC/Alamy

Yet something doesn’t add up. If younger consumers mean what they say about tackling the climate crisis, then fast fashion – clothes so cheap, they are practically disposable – should be going the way of plastic coffee cups. Fashion is the second most polluting industry in the world, second only to oil, creating environmental headaches right through from production to the landfill created when people tire of their bargains. (Even some charity shops are reluctant to take “value” brand castoffs because they are worth so little once prices are slashed for resale.)

Cotton is a thirsty crop, draining water resources, while polyester made from oil-based polymers has twice its carbon footprint. Deforestation caused by clearing land for clothing fibre production risks accelerating global heating. To protect the planet we should be spinning out quality clothes for longer, not constantly chasing the new, yet the rise and rise of Primark suggests a gap between what people say and what they actually do.

Just as voters will swear they don’t mind paying higher taxes for better public services, but then quietly vote for tax cuts, it’s possible shoppers know what they should do, but balk at paying more than they have to. “There has been a lot of justifiably bad publicity about ethical sourcing, paying people a pittance and all that,” says the retail analyst Richard Hyman, a former director of the retail intelligence company Mintel. “But I’m afraid the truth is that comfortable middle-class people may be able to adopt the moral high ground, but most people can’t.” And that’s where the gentle evolution under way at Primark gets interesting.

Behind a sliding door at one end of a changing room lies the Birmingham store’s “snap-and-share” room. Groups of friends can take in as many clothes as they want, set the lighting and music, and then film and photograph themselves on their phones before uploading it all to social media.

It is free marketing for Primark, essentially using customers as influencers, but it is also about putting sociability back into shopping. Hen parties love it, but so, apparently, do gaggles of older women, from a generation that grew up killing hours messing around a Boots makeup counter with their mates. The store, occupying the shell of an old shopping mall, is the size of a conventional department store, and designed to feel like somewhere you could linger on for lunch or to get your nails done in the in-store salon. Welcome to “experiential retail”, or shopping reinvented for people who want to do more than just click a button.

Facebook Twitter Pinterest The aftermath of Rana Plaza. The catastrophe put discount retailers, including Primark, in the spotlight. Photograph: Andrew Biraj/Reuters

“When people say the high street is dying, actually the high street is continually innovating,” says Tim Kelly, the company’s director of new business development. And while other brands are experimenting along similar lines, getting customers physically through the doors matters for Primark because the economics of selling online are not in its favour. When a £2.50 T-shirt could cost three times that to deliver, it needs the customer to come to the product, which means its stores must be enticing. But experiences are not the only thing Primark is trialling here. There is a recycling bin on every floor, encouraging customers to deposit old clothes for recycling or resale. There are free water fountains because the cafes don’t sell single-use plastic water bottles, and a new denim range made from sustainable cotton grown with less water, pesticide and fertiliser. For the festival season, the store is pushing eco-friendly glitter because the ordinary kind creates plastic pollution, and, while the greenest solution would probably be to stop wearing glitter, that is not the Primark way. The message is that you can still have fun with fashion, and they will take care of the guilt for you.

When the House of Commons environmental audit committee published a report on sustainable fashion last year, examining what big retailers were doing to limit their impact on the environment, Primark was ranked alongside M&S in its top category for engaging with environmental concerns. MPs’ sharpest criticism was reserved for the new online-only retailers such as Missguided or Boohoo, where a £5 dress can be driven to your doorstep for only £3.99 more. “The carbon footprint of people having it delivered to home, potentially wearing it only once for an Instagram, being pressured online by advertising – I think these models are psychologically more damaging and environmentally as damaging,” says Mary Creagh, the Labour MP and committee chair.

Facebook Twitter Pinterest The Birmingham store’s beauty salon. Photograph: Aaron Chown/PA

More than half of under-24s told a recent Mintel survey they now want to buy from labels that are kinder to the environment. “I think fashion is now where plastics was three or four years ago. People have woken up to the facts, but, because of the complexities of the supply chain and the competing claims of manufacturers, they can’t find their way through the ethical fashion maze,” says Creagh.

There is, she concedes, more Primark could do to protect overseas garment workers from exploitation in its infamously cut-price industry. But she thinks the Rana Plaza tragedy, which saw more than 1,100 people killed when a Bangladeshi factory used by retailers including Primark collapsed in 2013, was a wake-up call for the company and its customers.

Shoppers may not always have time to decipher labels, but they want to feel their brands are ethical, and in Birmingham everything from the “People. Respect. Planet” posters above tills to the branded brown paper carriers seems designed to reassure. All of which leaves the perennial question: if not by cutting corners, then how does Primark keep prices so low?

Kelly says the secret lies in doing things differently. “We don’t do big marketing campaigns,” he says. “We don’t have online shopping, or the cost of a delivery network that goes with it. We save on small stuff – our packaging is quite simple – and we do believe that all the way through our supply chain we do make a difference. Our focus has always been on our customer, on our pricing.” He won’t discuss profit margins, but Primark is a volume retailer, making up for relatively small profits on each item by selling lots of them.

It may sound suspiciously like corporate greenwashing, but Primark is, says Hyman, no cowboy operator. Having started in 1969 with one store in Dublin, where it is known as Penneys, it has grown its empire of 371 stores across Britain, Europe and the US slowly and cautiously. “It is a carefully managed business. It would be easy to assume that it’s ‘pile-it-high, sell-it-cheap’, that it involves a bit of ducking and diving. But it is not any of those things. It’s careful about what it does, and it knows exactly what it’s doing. It’s always been run as if it were a private company, owned by the people who run it.”

He sees some parallels with Aldi, the family-owned discount food retailer: “Aldi has always been much less cheap and nasty than people thought, and Primark is the same. People say: ‘How do they sell a T-shirt for £3?’ But most of its factories are pretty much the same as the factories that more expensive companies get their garments from. That’s the reality.” He thinks Primark is as least “as kosher as the rest”, even if that bar isn’t as high as some would like, given the inherent risks of retailers outsourcing production to distant countries where they can’t be wholly sure what is happening between inspection visits.

Facebook Twitter Pinterest The Disney cafe in the Birmingham Primark. Photograph: Aaron Chown/PA

No matter how ethically they are made, however, clothes cheap enough to wear twice before chucking them out will always have an environmental question mark over them. Primark’s defence to all this, aside from its recycling programme, is that it wants to see its clothes cherished. “We understand that there’s a long way to go,” says Kelly. “But from a Primark perspective, not everybody can afford a £50 shirt or a £100 jacket. We put a lot of work into our garments, and we want our customers to retain them for a long time.”

Yet the brand remains, for many shoppers, unashamedly associated with throwaway culture. Several of the women who let me snoop through their carrier bags in Birmingham are buying what Debbie McDermott from Coventry calls “cheap holiday stuff”: sundresses and sandals, things it doesn’t seem worth spending money on because they won’t get worn much. But a generation ago, our grandmothers probably weren’t buying pool slides, any more than they were ordering piles of dresses from online retailers just to Instagram themselves in everything once before returning it. Just as cheap flights have transformed our lives, opening foreign travel up to people on lower incomes, but encouraging the wealthy to take several breaks a year, cheap clothing has rather cheeringly democratised fashion and, somewhat more worryingly, helped us buy much more than we need.

The real secret of Primark’s success, says Hyman, is the hardest one for the rest of the faltering high street to copy: its extraordinary knack of converting casual browsers into buyers. “The magical thing they do, and there’s not a lot of it about these days, is when they get customers to go into their shops the customer feels almost compelled to buy something. People will walk around, and think: ‘Wow! I could wear that three times and throw it away, and still be quids in.’ And that wow factor is so difficult to create. It’s like magic dust.”

What strikes me only later is that while its competitors in Birmingham city centre seem busy enough, most shoppers still leave empty-handed. Outside Primark, however, customer after customer emerges clutching a carrier. Whatever that magic dust does, it’s working.