Along an Edward Hopper-esque cobblestone street two blocks from the Hudson river, outside a Brooklyn warehouse that is now a photographic studio the size of a baseball field, black SUVs are parked bumper to bumper. Inside, Mario Sorrenti, fashion royalty since he photographed a nude Kate Moss face down on a sofa for Calvin Klein’s Obsession in 1993, is perched on a wooden crate shooting the first Victoria Beckham X Reebok collection. Cara Taylor, the industry’s latest 17-year-old modelling sensation, a high school volleyball champ from Alabama whose fine curtain of wheat-gold hair falls across her cheekbones in the manner of a young Leonardo DiCaprio, wraps her arms around two other models, a boy and a girl, as Nonstop by Drake fades into All The Stars by Kendrick Lamar. Every couple of minutes, stylist Alastair McKimm, a Northern Ireland-born, New York-based godfather of luxe streetwear, darts on to set, minutely adjusting the hood of a sweatshirt with the beady eye of a society hostess plumping her drawing room cushions.

The designer of the Lucozade-orange trainers, sleek cropped tanks and oversized bomber jackets is perched on a director’s chair with a bird’s-eye view of it all. Victoria Beckham is wearing, as she always does, clothes from her catwalk label. Today, it is a military green sharp-collared shirt in stiff wool twill with shiny horn buttons tucked into matching high-waisted pleated trousers, accessorised with spike-heeled Balenciaga sock boots, a shiny red manicure and a bottle of San Pellegrino, which she sips through a straw so as not to smudge her lipstick. She hops down from the chair to pore over the monitors with Sorrenti or huddle with McKimm by the clothing rail. Beckham never raises her voice, but then she doesn’t need to, because everyone else stops talking as soon as she starts; she gives suggestions, rather than orders, but they are not queried.

You might think you’ve already heard a lot about Victoria Beckham, but if she has her way, you’ll be hearing a lot more. “I’m ready to put my foot on the gas,” she told me at her first-ever London catwalk show last September, and used the same line when she launched her YouTube channel last month. A decade after she moved into fashion, she has built a household-name brand, but the label has yet to turn a profit; there was a £10.2m deficit in 2017. “At the moment the brand is significantly bigger than the business,” she admits. “I want them to be the same size. That is 100% the plan, to reach as many women as I can.” Who’s her role model – does she want to be the female Ralph Lauren? She puts her head on one side. “You know who I was thinking about the other day? Donna Karan, and what she achieved.” Karan received a payoff in the region of $400m when she sold her companies in 2001.

‘I am behind the camera, then on the other side of it. Having to wear both hats can be stressful.’ Photograph: Nadav Kander/The Guardian

The potential of the Victoria Beckham brand made it an attractive enough prospect for David Belhassen (the investor behind the UK expansion of French bakery Paul) to put £30m into the business in 2017, a deal that valued the brand at £100m. While Beckham’s catwalk shows have been consistently well-reviewed – an initial tone of astonishment has faded – successful luxury businesses are not built on the market for silk day dresses at £1,500 a pop, which is tiny, but on the halo effect that catwalk glamour has on sales of underwear (see Calvin Klein) or lipsticks (Chanel), wallets (Paul Smith) and stationery (Kate Spade). Beckham’s Reebok collection will be joined by an “affordable luxury” beauty range later this year, which will be significantly cheaper than 2016’s Estée Lauder collaboration.

Of course what everyone wants to know about Victoria Beckham is what she’s really like. Her pop cultural character is equal parts Insta-perfection and tabloid gossip. When magazine coverlines about domestic meltdowns and diva tantrums run alongside glamorous images of Beckham and her photogenic family, it can be hard to make sense of the overall picture. But over the past few months I’ve hung around long enough to become part of the furniture; while our interview proper is scheduled for a couple of weeks after the Reebok shoot, I am here on set in New York, have travelled here previously to sit in on strategy meetings with Reebok, and accompanied Beckham to a Forbes women’s summit, where she announced the collaboration.

Designs from Beckham’s new collaboration with Reebok. Photograph: Mario Sorrenti

In person, the most striking thing about Beckham is how down-to-earth she is – a natural conversationalist with a knack for striking up an instant, low-level intimacy. Today, when I compliment her on her outfit, she pulls a face and whispers that she’s wearing a pre-production sample that doesn’t have the lining and is really itchy. She does this a lot, that thing that women do: deflecting compliments by saying something slightly self-deprecating. On the day of the Forbes summit, she arrived in the green room looking preternaturally polished in a gauzy powder-pink sleeveless dress; but when someone said how nice she looked, she was immediately off on a riff, talking us through the layers of unsexy nude underwear she’d had to put on underneath to avoid her nipples being the focus on stage.

She is skilful at dispelling the weirdness that celebrity brings into a room, which sounds easy but isn’t. Fame is a potent commodity that requires a smooth bedside manner to put others at ease, and a lack of this is why many famous people end up seeming odd or eccentric. Beckham may not have been gifted as a singer, but she has pitch-perfect chat, asking questions, finding familiar ground. When I ask if this is something she’s worked on, she says, “There’s always so much to do that there’s really no time for bullshit – so that cuts through it.” When we meet again at Guardian Weekend’s cover shoot in London, two days after the British Fashion awards, she arrives at the studio and asks if I had fun (we had caught up briefly), says how charming Meghan Markle was on stage, and commiserates about the pain of a late finish on a Monday night (“At 1am I was thinking, I’d love to stay but I’ve got to be up with the kids at 6.45”).

People are interested in my personal life, and sometimes it’s things I don’t like. I’m not going to let it get me down

She pays compliments, cracks jokes and is not above sharing juicy celebrity gossip, albeit followed up swiftly with a “that’s off the record, obviously”. The assistants and outriders who accompany her everywhere are a tight-knit, mostly female, fiercely loyal team, but she does not use them as a human shield in the way that many celebrities do. A recurring theme in the media narrative about Beckham is that she doesn’t smile. There is perhaps a touch of misogyny in this tabloid obsession; you don’t see famous men vilified for not grinning in the Heathrow arrivals hall. In real life she smiles a perfectly normal amount.

Being Victoria Beckham is a 24-hour, seven-day-a-week job, one that is both pampered beyond your wildest dreams and brutal. Take, for example, two days we spent together last May. Having flown from London to New York on an evening flight, posting hotel lobby selfies at late-night check-in, Beckham arrived for morning meetings with Reebok serene and elegant in a pale silk striped shirt dress – the glamour a necessity not for the meeting, but because the paparazzi wait for her outside every hotel and images of Beckham wearing her own designs are a key sales driver.

Victoria Beckham does not design the detail of every dress herself, but rather is a hands-on creative director to the design teams, a modus operandi that is not unusual in fashion. The cynical view of her as a mere frontwoman does not stack up. In that meeting she skippered the ship: offering ideas, making judgment calls, staying focused in the boring bits. I started to forget she was famous, until her phone buzzed and a message bubble from “David” popped up on the screen. She reached out a manicured hand and discreetly turned the phone face down. This was barely a week after rumours of a Beckham divorce announcement brought the internet to virtual meltdown – more of which later – yet the image she presented was of bulletproof serenity.

With all the family at new year. Photograph: Digital/eroteme.co.uk

That evening, after meetings had finished, we had a drink in the bar of her hotel with her assistant, two of her PR team and her CEO, Paolo Riva. She decided her favourite order of tequila and soda wouldn’t mix well with jet lag, and had sparkling water while we discussed talking points for the Forbes event the next morning. But the jet lag still hit her with a vengeance, and she was up at 4am posting Instagram photos of herself in Korean sheet masks, before appearing fresh and smiley for her on-stage turn. She fired a deft tranquilliser dart into the elephant in the room with several warm references to the supportive husband currently “at home in London doing the school run, cooking dinner, helping with homework so that I can be here”. Within minutes of walking off stage she was fine-tuning the schedule for the following day with her team, juggling appointments to be able to pick up her seven-year-old daughter Harper from school and take 16-year-old Romeo to tennis on her one day in London, before leaving to take her new collection on selling appointments to buyers in Paris. We travelled home overnight, on different flights. The next morning I was on my way to the Guardian office in jeans with hair still wet from the shower when I saw a chirpy Instagram post of her – hair perfect, makeup done – trying on dresses for her meetings. Daft as it sounds, with me jostling my way on to the tube and her about to step into her chauffeur-driven car, for a minute I almost felt sorry for her.

Being Victoria Beckham requires her to be both front of house and behind the scenes. On the Sorrenti shoot, after 15 images of various combinations of three models have been taken to everyone’s satisfaction, interspersed with outfit changes and snack breaks, it was time to shoot publicity images of Beckham. That’s quite intense, I say to her afterwards. “Yeah, it is. I am behind the camera with Mario and Alastair all day, then at the end I am the one who has to be on the other side of the camera. Having to wear both hats can be stressful,” she admits. Immediately, though, the lessons learned after 25 years in the public eye kick in, and she autotunes herself. “But I’m not complaining. I have an incredible life. I know how lucky I am.”

From the Reebok collaboration. Photograph: Mario Sorrenti

She looks quite ridiculously good for 44 years old. Not just groomed and polished, but with luminous skin and sparkly eyes. She is extremely slender – not starved-looking, but very delicate, as if built from wishbones. At home she works out for two hours a day. She gets up between 5.30am and 6am, so that she can get some of her workout in before the school run, starting with 7k on the treadmill, “a mix of uphill fast walking, jogging, running. That’s the only time I watch TV – boxsets, documentaries – so I look forward to that. It takes 45 minutes. Then I work out with a trainer – 30 minutes legs, 30 minutes arms, toning and conditioning, then loads of planks and that kind of thing for my core. At the weekend I will do the whole thing straight through, but in the week I often jump in the car halfway through to take the kids to school, then carry on when I get home. I work out every day when I’m at home, and then when I travel I really focus on work so I can get as much done as possible in a short trip and get home.” Working out is, she says, “a really positive thing for me. It’s part of who I am now, and I really enjoy it. That was a big part of why I wanted to work with Reebok – I had very specific ideas about the workout clothes that I wanted to wear and couldn’t find.”

She doesn’t pretend to be eating as many cheeseburgers as she likes. From the lavish catering table at the New York shoot, she chooses some smoked salmon and salad. She carries flavoured sweeteners, which she pulls out of her handbag and squeezes into black coffee. “I never cook. I used to – when we lived in Spain, I used to cook a lot. But these days I don’t tend to get home till late, so dinner wouldn’t be ready till quite late. David’s a really good cook.” You don’t get the sense that food is exactly a passion. “I am very, very disciplined in the way that I work out, in what I eat. That’s how I’m happiest. I expect a lot from my body – I’m 44, I’ve got four kids, I work a lot, I travel. For me to do all that, I have to eat healthily and work out.”

Beckham talks about her famous husband and now-almost-equally-famous children all the time. The Reebok launch event was initially slated to happen in Tokyo, and during a meeting discussing possible venues, she suggested restaurants that David and Brooklyn, aficionados of Japanese cuisine, had loved. When we look at the collection in New York, she proudly shows me the pieces that 13-year-old Cruz, a streetwear obsessive, has earmarked as his favourites. Lunch break time in New York is teatime in London, and when Beckham takes her phone into a corner to FaceTime with home, the voice of Harper booms out, excited to tell her about going to Winter Wonderland with Daddy.

The Victoria Beckham brand and the Beckham brand share an ecosystem, and she is matter-of-fact about this. “People are interested in my personal life. And sometimes that works in my favour and sometimes it’s things that I don’t like. I’m not going to let it get me down.” When the children were little, she used to tell them that the photographers were there because their grandparents missed them and wanted to see pictures of them. “It was a way of explaining it to them when they were too young to understand. They are used to it now. They’ve all grown up with it. They understand that they have to act in a responsible way because of it. But they will make mistakes – we all do. And theirs will be in the public eye.”

‘People do believe what they read, and when it’s completely fabricated, that’s really annoying.’ Photograph: Nadav Kander/The Guardian

Beckham learned about clothes through wearing them and being photographed in them – a background that was sniffed at when she launched her first collection in 2008, but that has come to feel increasingly relevant in a world in which individual experience is paramount and piety toward expertise a quaint concept. Her catwalk brand has evolved from elegant but somewhat overwrought dresses – the sort of thing you need if you go on a lot of dinner dates in Mayfair – into fluid, confident day-to-night tailoring. A voguish narrative of female empowerment in her clothes mirrors her own journey from manufactured pop star and trophy wife to respected industry leader. Chart-topping bands know how to connect with a mass audience, and Beckham is skilled at breaking down the fourth wall, talking about style in a way that feels authentic and relevant. “I wear my clothes, I travel in my clothes, so I really care about what something looks like when you get off a plane or pull it out of a suitcase,” she says to me one day in New York when we are discussing the challenge of a business trip wardrobe that needs only carry-on luggage. Of the beige shade that features in her Reebok collection, she says, “I love the biscuit colour because I wanted workout clothes in a palette that would work with the coat I’d be putting on over the top.” She pulls a sweatshirt off the rail and enthuses about the size of the hood, and the extra length of the drawstring. “A hood has to be the right size, otherwise it looks awful, and the drawstring needs to be long enough so it doesn’t come out, because that’s so annoying when it disappears.”

On the catwalk at London fashion week in September, talking to son Brooklyn (in cap). Photograph: WWD/Rex/Shutterstock

Family life in Holland Park sounds nonstop. The Beckhams’ second eldest son, Romeo, appeared in a couple of Burberry campaigns in 2013/2014, but now spends most of his free time playing tennis. Cruz is “in and out of the music studio, loves to play guitar and piano, write and record songs”, while Harper is “absolutely obsessed” with ice-skating. (“It’s very I, Tonya,” Beckham deadpans.) Brooklyn, 19, “wants to be an art photographer, and also enjoys fashion, so we chat a lot about that”. David Beckham’s ownership role at Florida’s newly launched Major League Soccer team, Inter Miami CF, has prompted speculation of a move to Miami, but Victoria says this won’t happen. “It’s a fantastic city, and David and I are partners in every business venture that we have. We have made some great friends there and socialise with their families, so we will go regularly. But the kids are happy at school, we are close to our families and we love being in London.” Next month, for the second season running, her catwalk show will take place in London fashion week rather than New York, which means less travelling. “A project like Reebok is international, so there is travelling involved. But it’s workable. I mean, the emails from the kids’ schools come on my phone wherever I am, I can still deal with stuff. And, don’t get me wrong, I have help. I have a cleaner, so I don’t have to wash or iron. And I have someone who helps me with the children, who is wonderful and who I trust implicitly. David does a huge amount of travelling, and that’s so hard on him.”

For years, the Beckham marriage has been a national obsession. When she appeared on the cover of British Vogue with their four children last autumn, shortly after the divorce rumours, the noise was all about the absence of her husband from the newsstand cover, rather than about her 10-year anniversary in fashion. How does she cope when the parallel narrative about her marriage takes over? She fixes me with an unblinking stare, and the temperature in the room seems to drop a degree. “It can get quite frustrating,” she says coolly. “But I leave it to my PR team. I don’t get involved.” Then she tells me a story about a magazine interview that had reported her demanding food from the Ivy when “I had a plate of fruit from M&S, which my assistant went out to get”. It is a firm steer away from David. All she will say is that “you do have to be quite controlling because people do believe what they read, and when it’s completely fabricated, that’s really annoying”.

I’m sure when they are on stage, a part of me will feel a bit left out. Because a part of me will always be a Spice Girl

For the record, I have no idea of the state of the Beckham marriage. I think most of us know, really, that the private reality of a 20-year marriage cannot be gauged through internet gossip or a stage-managed photograph. But it seems to me that much of the conjecture about the marriage is really about Victoria’s public evolution from being the lesser talented Beckham into a successful woman in her own right.

With the Spice Girls in 1998. Photograph: Mike Prior/Redferns

The new Reebok collection will make Beckham’s audience more unisex, more diverse, more youthful. She is looking for ways to move the focus away from her. Being her own shop window has been an effective and economical marketing strategy, but she can’t be in every global market at once. At the recent British Fashion awards, she hosted a table of women including Brooke Shields and Gia Coppola, who wore her clothes for the event. “Having them flying the flag with me means that it’s not all on me, you know? I get quite obsessive about work. I don’t stop, I have a lot of trouble sleeping. I am a real insomniac.”

One thing Beckham won’t be adding to her schedule is a Spice Girls comeback. When the band return to stadiums in May, it will be as a foursome. No cameo appearance, I ask? No hologram? “No. Definitely not.” Was that a difficult decision? “Not at all. What I do now is my passion and a full-time job. I’m excited to see it, though. And I’m sure when I’m there and they are on stage, there will be a part of me that feels a bit left out. Because even after all this, a part of me will always be a Spice Girl.”

All clothes: victoriabeckham.com. Styling assistant: Bemi Shaw. Hair: George Northwood. Makeup: Valeria Ferreira. Shot at icetank.com

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