Turning the other chic: Will real men wear Matthew Williamson's new menswear range?

He has made fans of Kate, Sienna and the high street with his bold and bright boho designs. Now he's turning his sights on the other half of the population with his first menswear range

Matthew Williamson is leaning against an empty shop front near his studio in Mayfair, having his picture taken as a feeble sprinkling of snowflakes falls from the chalky grey sky. For a fashion designer whose name is synonymous with jet-set, pool-to-party living and sun-drenched places populated by the beautiful and the tanned, the urban location (even one as uptown as Shepherd Market) and grim climate seem all wrong. But, for the moment at least, the gap between the reality of Williamson's daily existence and the exotic lifestyle fantasy on which he has built his brand could not be wider.

"I'm working from eight in the morning till 10 at night every day right now," he confesses in his even Mancunian drawl, sounding, if not cheerful, then matter-of-fact about it all. "I don't really have much of a life, I really don't. I went out last night for the first time in god knows how long. I go home, eat my dinner and go to bed. It's not how I want to live for ever; it's not healthy to be this consumed by your work."

Still, Williamson is hardly your average rat-race drone; his appearance alone is confirmation of that. Even wrapped up for the cold in a thick winter coat, trousers tucked casually into chunky boots and a bright scarf at his throat, he is what my grandmother might approvingly describe as "immaculate" with his razor-sharp cheekbones and clear skin.

And if Williamson isn't exactly thrilled by his current hours, his passion for the work has not dulled. Just as well, given that there is a lot of it on the 38-year-old's plate just now. Besides the frantic final preparations for his autumn/winter 2010 womenswear collection, which he will show at London Fashion Week later this month, he has a separate beach and swimwear range in the pipeline and a book celebrating his career set for publication later this year. Before any of that, however, there is the small matter of the first Matthew Williamson menswear line, launching tomorrow at Harrods.

"This is just us dipping our toe in to begin with," says Williamson of the inaugural range of cashmere jumpers, printed T-shirts and scarves. "We'll see if people are interested, then hopefully we'll build on it." For all his cautiousness, Williamson's track record augurs well. His eponymous womenswear label, a favourite at the young, nubile end of the celebrity spectrum (think Sienna, Keira, Mischa), is now in its 13th year and boasts stand-alone stores in London, New York and Dubai.

Beyond the catwalk and the red carpet, Williamson has proved himself capable of catering to the high-street masses too, with a long-running diffusion line for Debenhams (a bold move back in 2002, when designer collaborations with chains were not the norm they have since become) and, last summer, a high-profile hook-up with H&M. It was the project with the latter, which included a collection for men as well as women, that planted the menswear seed in Williamson's brain.

"I've always wanted to do menswear – it's never been a struggle to think about how it would look, but I guess H&M is the reason for why now. That was a really nice project, as I got to design clothes I could wear myself. And it sold well, so I was chuffed. It inspired the thought that it really could happen."

The idea crystallised when Harrods' buying director, industry legend Marigay McKee, approached Williamson "with a vision she had of rainbow-coloured men's jumpers". Her choice of designer is apt, given the signature palette of vivid hues and bold prints Williamson established with his breakthrough Electric Angels collection at London Fashion Week in 1997.

That show, which featured Kate Moss, Helena Christensen, Jade Jagger and the then-unknown Diane Kruger sashaying down the runway in neon dresses and separates embellished with exotic-looking feather and butterfly designs, made the St Martins graduate's name and introduced the two things with which the designer has been most closely associated ever since: kaleidoscopic creations and gorgeous, famous female friends willing to flaunt them.

But does he think British men are really ready for flamingo, chartreuse and aquamarine? "It's funny; I looked at all the little bits of advance press that the collection has got on the internet and, without exception, everyone goes, 'Ooh, bright colours.' But there's eight jumpers and this is as bright as it gets," he says, tugging at the deep-purple fine-knit V-neck he is modelling.

"There's an oatmeal, an olive, a sky blue... there are quite sober day-to-day colours in there that my business partner might wear under a suit. I think everyone expects it all to be colourful, because that seems like the obvious thing from me, and yes, there's an apple green and magenta. But that's not to say there aren't more muted shades too."

Others' perceptions of what might be the "obvious thing" for Williamson are clearly something with which the man himself has been grappling for some time. For although his early collections were hailed as a breath of fresh air in the grungy late 1990s and he sailed into the noughties on the new "boho chic" wave, a wider turn in fashion towards leaner, meaner looks in recent years cut him slightly adrift in the design sphere.

"You get pigeonholed, you know?" he says. "I think people think I just make pink chiffon dresses. They think of Jade Jagger, they think of Ibiza... all of which is true and, on the one hand, that is brilliant. If people can say what I do in five words, it means I've built something recognisable and I'm proud of that. But creatively, it's suffocating. I want to turn a corner and I've really worked on that with my last two womenswear seasons. I want the collection to have the same DNA that it's always had, but to evolve."

His spring collection – presented in London last October, marking Williamson's return to the capital's fashion week after a seven-year stint in New York – was noticeably sleeker and more urban than the nouveau-hippie chic of old and, as Williamson points out, it wasn't all about neons either: "The bestselling piece from that collection is a little black dress [inset right]. It's interesting because it's kind of the antithesis of what I am known for, but it's a colour that is always in my collection somewhere."

Williamson has too much sang-froid to sound bitter, although it might not be unreasonable for him to feel a little hard done by. While Williamson and his business partner (and ex-boyfriend) Joseph Velosa have managed to successfully grow the label into a global brand without selling out to the corporate behemoths who have snapped up other small British design houses, Williamson has been sidelined by snootier parts of the fashion press as the king of kaftans, a one-trick pony. It's partly this that has driven him to create the forthcoming separate beach and swimwear collection:

"Just recently someone wrote in their review of my autumn pre-collection, 'Where's the kaftans?' Well they're there, I still design that stuff, but I don't necessarily want to put it on the runway any more as a statement."

I ask whether he thinks there is a snobbery within European fashion that dismisses clothes that are unapologetically pretty and cheerful as simplistic and uninteresting? "I was re-reading a bit of the text from the book I'm working on the other day and there was a quote from me years ago about St Martins and the fact that I didn't fit in at the time, or I felt I didn't, because I wasn't visiting lunatic asylums for inspiration. I just wanted to make great-looking clothes. There is definitely a sense that if you haven't obviously tortured yourself to produce your work, it's of less value.

"The work that I do is, I think, incredibly complicated. It might look effortless or flighty but it's far from it. It takes a bloody big, very skilled team and a lot of hard work. It's as complex as the next designer's work. But different people have different perspectives on what you take your inspiration from. You can't please everyone, you just have to do what you do and hope that your vision works. There's nothing better for me than knowing that I've produced a collection that women want to buy."

Ploughing his own furrow is Williamson's default setting anyway. Growing up in a Manchester suburb, he knew he wanted to be a fashion designer before even hitting his teens and, with the help of his "fabulously supportive" mum and dad, moved to London to become the youngest student admitted to St Martins at just 17.

"I had dogged ambition and single-mindedness almost to the point of being quite weird," he says, and I believe him. Maybe it's just genetic coincidence, but everything about Williamson – from his upright posture to his pin-thin, yoga-honed physique and impossibly symmetrical features – exudes discipline and self-possession.

Having only ever seen him unsmiling and fashionably patrician in photographs before our meeting, I expect him to be a little cold. But beyond the natural physical hauteur, he is quietly friendly and polite. The fact that he is not especially effusive makes him seem all the more genuine and it's easy to see why so many famous women go to him for clothes – Williamson doesn't seem the kind of guy who would be drawn into the swirl of fussing and fawning that celebrities attract.

More importantly, he seems as though he would be tactfully honest if a dress made you look like a sack of potatoes – although I suspect that may not be a huge concern for close friend Sienna Miller, who Williamson says is, besides his mother, the person he most likes seeing in his clothes.

Who would he like to see in his menswear? "Well, I gave Jude Law one of the jumpers, so I'd love to see him in it. Mika, I think, would look good and Johnny Depp is probably the most stylish guy I can think of, so he'd probably be the dream."

But if Williamson's ambitions for the range are typically lofty, his dry humour is always on hand to puncture any impression of arrogance: "I guess I am just reflecting myself in the collection and I'm not the kind of guy who dresses head to toe in black and grey – I like little pops of colour. So I hope people feel the same as I do. Well... I'm screwed if they don't."

The menswear collection will be available from tomorrow at Matthew Williamson, 28 Bruton Street, London W1 (tel: 020 7629 6200), matthewwilliamson.com/shop and Harrods, 87-135 Brompton Road, London SW1 (020 7730 1234)