Matilda, 18, has her holiday wardrobe sorted. It’s all been done in a one-stop retail hit. Bikini, beach cover-up, cotton kaftan and a little black dress. It’s a capsule wardrobe, as fashion editors love to say. The whole lot cost her less than £35 in Primark and she is thrilled.

“It’s funky and cheap,” she says. “I love it.” She particularly loves a bargain because she babysits for a tight-fisted family in Hammersmith and she’s desperate to save up for her gap year.

I know because I am that stingy Hammersmith mother. For the past few months I have been astounded as Matilda has arrived on my doorstep looking chic and up-to-the-minute. First it was a furry gilet, then a military-style jacket followed by a gypsy skirt. Meanwhile I am getting last year’s summer clothes out of the attic. How does she do it? She’s only just left school, for heaven’s sake.

The answer for her, as it is for thousands of shoppers across the country, is Primark. Once a little-known discount retailer, the clothing chain is fast becoming a high street phenomenon. It is the first stop for those with an eye for under-a-fiver bargains — and a pair of pointy elbows to shove other shoppers aside. Not only does it pile it high and sell it cheap, it can change its fashion looks daily.

Primark is the high street’s flirty little secret no more. Last week its owner, Associated British Foods, announced that it had bought 120 Littlewoods stores for £409m. Of those, 30 will become Primarks and it plans to increase its presence in the high street by 50% over the next two years. Operating profits have risen by 18% to £59m in the first half of this year.

At a time when the middle market, led by Marks & Spencer and Monsoon, is feeling the squeeze, the low-cost sector is booming. Worth an estimated £6.4 billion a year, it accounts for nearly 20% of the total clothing market, compared with 11.9% five years ago. In terms of the value retailers, Asda leads the way with its George brand, followed by Matalan, Primark, New Look, TK Maxx, the Peacock group and Tesco’s Florence + Fred, Cherokee and Value. Two years ago only one in 10 customers patronised such shops, now it is one in eight.

It has become quite the thing for the middle classes to share their giddy, guilt-free Primark moments. Snaffling a Chloe lookalike handbag, for example, for only £4 when the real thing costs a hundred times more. This strikes something of a blow for marital harmony. Arrive home after a day’s shopping with bags from Jimmy Choo and you may meet resistance. But come back with Primark carrier bags and you can say: “Look, darling I saved you at least £1,000 today.”

It’s tempting to say that the clothes are as cheap as chips, except that would be unfair to the potato. Considering how far some of the £1 and £3 items have flown (from India, Mauritius or China), a bag of chips works out much more expensive.

“I go there for regular retail therapy,” says Lucy Peck, a 43-year-old mother of three, whose regular visits to Primark after dropping her children at school threaten to become an addiction, though one that is hardly going to break the bank. “The stock changes all the time, you zip down there and buy it. And if you make a mistake, who cares about a tenner? I take my daughters there all the time for treats.”

It is clear for fashion sophisticates such as Peck, who lives in a leafy west London suburb, that shopping in the value sector no longer holds any stigma. After Vogue featured a £12 jacket in its May issue, the scene was set for fashion meltdown. The jacket flew out of the shops as did sequined shrugs for £10 and camisoles for £2. Who cared if the shrug was worn once and then, er, shrugged off? This is disposable fashion at painless prices.

Claudia Croft, fashion editor of The Sunday Times Style magazine, believes Primark smashed through the class ceiling this year with three breakthrough garments. “They produced the now-famous military jacket, then the sequined shrug, followed by the gypsy skirt,” she says. “They are brilliant at fast fashion, copying looks off the catwalk and putting them in the stores quickly. They are roasting the competition.” Many people, she adds, make the pilgrimage to Primark stores, not always easy to find, several times a month. “I think its success lies in its ability to sell several things to one person, particularly accessories. It’s not about selling one garment to one person.”

So what is the experience like? Well it’s not like walking into Prada in Old Bond Street where a gentleman opens the door for you and bids you good morning, madam. Hit Primark at the wrong time (after 9am) and you are likely to find long queues for the changing room and till, and items strewn on the floor. Its store decor is basic and, dare I say it, the lighting in the Ritz restaurant is more flattering.

So what is happening in Britain’s high street? It’s a topsy-turvy world when your lunch from Pret A Manger (sushi, avocado salad wrap and carrot cake) can cost more than the staples of your holiday wardrobe. Competition among retailers and cheap imports from the Asian giants such as China and India have driven down profit margins and prices, but there is more afoot than the economics of scale. The nation’s sartorial and shopping habits are changing.

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