These are lean times for retailers. The modern consumer rarely needs to go shopping – rather, he or she wants to go shopping; and once recession bites, retail therapy can look about as desirable as that never-used gym membership.

Never before has the high street needed someone like Mary Portas.

Portas, most familiar now from her BBC2 show Mary, Queen of Shops, made her name when she turned around the fortunes of a fusty little department store in Knightsbridge, London, called Harvey Nichols in the early '90s. In her last TV series, she performed the same magic trick on shoddy shops up and down Britain.

Her transformation of Harvey Nichols was no mean feat. Back then, the UK was in the grip of a recession – and she got people into the shop by using nothing more complicated than window displays. One of her most famous displays used mannequins to recreate famous pop bands. "One woman said to me: 'I've got no idea what those windows are supposed to mean, but I feel fantastic going in the shop.' And that's all I wanted."

Portas lounges at a conference table at the central London HQ of her retail consultancy, Yellow Door. She's dressed in skinny jeans, a white shirt, a long, multi-stranded necklace and a ring the size of a chicken's egg on her right hand. She talks very fast and is at least six feet tall in her high-heeled boots. Her trademark sleek auburn bob, cut at John Frieda, gleams in the sunlight. She stalks and slinks through the room graceful as a greyhound, beady as a magpie.

"At times like this, most retailers panic and get pulled down into this black hole of fear and can't see a way out. I think that during the last recession people loved the fact that there was this bonkers shop on a corner of Knightsbridge that was all joy. Retailers just need to keep upbeat." The stores that will survive a recession, she says, are those which are either bargain-basement or selling top-end luxury: to the cash-strapped consumer, anything in between just isn't value for money.

Portas is a natural on television; it makes sense that once upon a time she wanted to be an actress. She is tough on the hopeless shopkeepers ("You don't do any particular thing well here"), throws out suitably catty asides to camera (holding a hideous lace dress: "God almighty. Imagine the wind up your parts wearing this round Cardiff") and looks a million dollars at all times. She styles herself, of course. This is a woman who never, ever wakes up and thinks, "I've got nothing to wear."

It makes marvellous TV. Although it is (as she says grimly) "genre" television, it appeals to the viewing public's penchant for schadenfreude and fondness for colourful characters who've got answers. One episode of her last series was set in the Ascot boutique Blinkz, run by Amanda (a size 10 fitness instructor), which caters for ladies of a fuller figure. The only problem is that Amanda doesn't like fat people – ie, her customers. "They don't like clothes made out of a thicker fabric because they sweat more," Amanda says blithely to camera. Portas's eyes saucer and her face falls as if she's been shot. "Oh my god, she's sizeist," she whispers urgently to the camera.

And now it seems that the public have taken to Portas's tough-love as much as they have to Super Nanny or Gordon Ramsay. "I think I'm perceived as a confident woman, possibly tough, although actually I'm quite emotional," she says. "I'm strong and no-nonsense, but I'm also a super-sensitive family person. I've never wanted to be that sort of ball-breaking woman who sacrifices family or family happiness for her career. No way!"

In the past, the shows have been about Portas going to save failing retail business. This week sees a new show, this time on Channel 4, where Portas looks at shopping from the customers' point of view to see if we are being served. "I just think we've lost the whole culture of service. I think we've ended up with sales teams stacking shop floors as opposed to any type of service. I think it's criminal," Portas says. "There's now a whole generation – certainly my kids, who are teenagers – who don't even expect good service. They don't even expect to be greeted now when they go into a shop. Now, when you go to the supermarket, it's just beep beep, 'Have you got a clubcard?' And that's it. They don't even tell you the price – you have to look at the machine.

"As consumers we just accept it. So I'm going to go into businesses and ask them 'Are you genuinely putting the customer first?' You look at the cultures of these businesses, and you realise nobody on the shop floor is being properly trained or given time, and they're the ones who are the interface with the customer."

It emerged after her television debut that Portas, who was married for 13 years and has two children, Verity (15) and Mylo (16), is now in a relationship with a woman: Grazia magazine's fashion editor at large Melanie Rickey, whom she wed last year.

One of her greatest achievements, Portas has said, was negotiating an amicable divorce from her husband, Graham, a chemical engineer. Indeed, with her career and her cosy domestic situation, she does seem to have managed to crack the modern female dilemma of how to have it all.

"There have been times when I've thought, 'God almighty, lie me down for a week and no one talk to me.' Or I've looked in my diary and thought, 'How am I going to clear all that?' But I genuinely think I've been able to both run my family and a successful business. I am so proud of my children. I know they get the most of me and the best of me and I get the best of them."

One of five children, Portas was born in 1962 in Hertfordshire, England, to Irish parents; her mother died when she was 16 and her father two years later. Her elder siblings were at university or working abroad and she was left on her own with her then 14-year-old brother Laurence.

"It was pretty hideous," she says. "I just somehow had to manage it all and not crumble. I look at my son Mylo and I think, 'Oh my god, that's how old Laurence was.' It makes me very upset. And at the time, I just didn't know how young we were. But we got on with it – and I believe it was the making of me. Being in that situation and having to cope with it all made me think: right, this is your world now, Mary."

The Portas family ploughed on through their hardship. After a few false career starts (neither acting nor art school agreed with her) Portas got a part-time job at Harrods, after stints at John Lewis and Topshop, working on the window displays. The part-time job turned into a full-time job and three years later she joined Harvey Nichols. As well as realising the new Harvey Nichols owner Dickson Poon's vision for a modern, upmarket department store with her far-out window displays, Portas pulled off a stunning PR coup: the store featured prominently in the hit comedy series Absolutely Fabulous. As a result, everyone wanted a piece of Harvey "Nicks".

Portas's real success stems from her partnership with the international marketing genius Peter Cross, with whom she runs Yellow Door. Louis Vuitton, Miss Selfridge, Oasis and French Connection have all beaten a path to their door, wanting a touch of what Portas calls "pixie dust" – the magic combination of elements that turn a shop from dump into destination.

What kind of problems does an established brand such as Oasis have?

"I talked to Oasis mostly about focusing on the fact that they are only for women and they're a feminine brand and that they should develop that and separate themselves from something a bit more urban, like Topshop. I don't look at the product. I look at what the brand means to the consumer."

It's been a tough 10 years for brands like Oasis. It was, along with Kookai, Next and French Connection, once the go-to shop for an 18-year-old with her allowance burning a hole in her pocket. Then came the triptych of Topshop, H&M and Primark/Penneys. Suddenly not just the 18-year-olds, but the 25-year-olds and 40-year-olds, started abandoning the big hitters of the high street for these shops that sold clothes, of variable quality, at knock-down prices.

"H&M I don't mind," says Portas. "There's a very sophisticated design team there and there always has been. You look at some of their stuff and you think: that's clever. It's when a shop is just turning the stuff over without a care for design, the environment or about selling. It just gets to me and it knackers the retail trade. I don't like it."

By this she means Penney's, or Primark in the UK, which was the subject of the Panorama programme Primark: on the Rack, which exposed the shop as selling clothes produced by child labour.

"I don't know what's going to happen to Primark," she says. "I do point my finger at the fashion press for helping Primark become as big as it is. All that [puts on a snivelling voice] 'Primark is the new Prada' and 'Primarni' stuff is not funny.

"What's interesting about Primark is how many middle-class people are in there, buying for their kids. I've walked through Selfridges on a busy Saturday and all you see is those brown Primark bags going up and down the escalators. So all that stuff about Primark being democratic fashion for people who can't afford anything else is bollocks," she says, jabbing a finger into the air and narrowing her eyes.

"We've become so fast in the fashion industry that we've lost what fashion is all about. There is no anticipation of the season – there isn't really a season anymore. It's just Big Mac fashion.

"There's too much product out there, too much choice – and it's exhausting. I believe that there will start to be more specialist shops emerging; it's already happened to coffee!"

She pauses. "This is all just my feeling, though. None of it's proven." She starts laughing. "Ten years down the line someone's going to pull this piece out and say, 'Well she got that bloody wrong, didn't she?'"

Somehow, I doubt it. They wouldn't dare.

'Secret Shopper' begins on Wednesday 19 January, 9pm, Channel 4

Mary's tips on how to shop

Sign up to your favourite stores' online newsletters: that way you'll be the first to know about their sales, new collections and special offers

Do your research online before you leave the house. Don't be afraid to call ahead and have things put aside

It's very easy to be seduced by all the lovely new season wear, but keep an eye on the sale rails, which might just have that gem to complete your look come next season

It's not a bargain if you wouldn't have wanted it at its original price. Don't buy things just because they're on sale

Be careful about shopping on holiday. The floral poncho that looks so great on all the locals might not translate back home

The coolest time to shop is first thing in the morning, when both you and the staff are at their best

Don't stand for bad service. If they're rude, lazy, ignorant or all three, they don't deserve your custom