Celia Birtwell in her West London home © David Sandison
Her designs for Topshop smashed high-street sales records, and the clothes she created with her husband Ossie Clark remain the epitome of Sixties' style. Now Celia Birtwell is back – with a dazzling new collection that shows she's still at the top of her game. Susannah Frankel gets a sneak preview

I grew up with a lithograph of a David Hockney portrait of Celia Birtwell hanging, in pride of place, on my parents' sitting-room wall. Birtwell stands looking at a potted tulip on the table in front of her, one foot forward, hand in pocket, wearing a loose-fitting trouser suit and high, strappy sandals, and trademark chiffon scarf tied neatly into a pussy bow at her throat. There is the instantly recognisable head of short, tight curls and just a hint of a smile on her Cupid's-bow lips – the term might have been invented for her.

The image is very spare but, as might be expected given the hand behind it, is a great likeness nonetheless, shedding at least some light on the woman who, with her husband Ossie Clark and, of course, Hockney himself, was the toast of le tout Londres, and indeed the world, for the decade between the mid-Sixties and Seventies. Birtwell's iconic status aside, here is a woman who exudes an unlikely mix of the imposing and the sweetly feminine, the earthy and the slightly flighty, the mischievous and the angelic, the matriarchal and the eternally girlish. More than anything, however, the picture is of a woman in control, although the subject herself would probably be mildly irritated by such analysis. "I just always stand with my hand in my pocket and my foot forward," she'd say.

Next month, almost 40 years since that portrait was painted, Celia Birtwell's fourth collection for Topshop will arrive in stores across Britain. It has been the chain's most successful designer collaboration to date – and that includes the hugely publicised line put together in partnership with Kate Moss. Birtwell has recently signed a similar – "bigger" – deal with the American high-street chain Express, that goes on sale in the spring of this year. She has also designed prints for everything from wellington boots and cagoules to tepees and umbrellas for Millets, due to launch just in time for Glastonbury. Her daughter-in-law Bella Clark, wife of her youngest son George, is a keen camper, she says, and while the colour mix is "a bit less bright than I'd normally use", this is because "they blend in with natural surroundings".

"Topshop came along, and from that time on there has been an awful lot of interest," Celia Birtwell says, modestly, over a cup of jasmine tea at her home, a few minutes' walk from Notting Hill. "I felt this swell of interest, which was really nice for me because I would never have imagined it. It was thrilling."

Perhaps not quite so unreserved are her feelings about next week's London Fashion Week relaunch of the Ossie Clark label without her input or, of course, that of her late husband, who was killed by his lover, Diego Cogolato, in 1996. Marc Worth, co-founder of the fashion-industry website Worth Global Style Network (WGSN) has bought an exclusive licensing deal from Alfred Radley, the clothing manufacturer and high- street retailer who purchased Ossie Clark's company from the financially struggling designer in 1968. The relationship between Birtwell, Clark and Radley seems to have been strained. Radley paid both designers to come up with ranges that he went on to sell in large volume to a mass market, and Clark, in particular – whose passion lay in creating elaborate one-offs for the beautiful people – found it difficult to give up complete control.

Nonetheless, Radley owned the Ossie Clark name and duly sold in to Worth, who has now employed the designer Avsh Alom Gur, formerly of Donna Karan, Roberto Cavalli, Chloé and Nicole Farhi, to revive it.

It is entirely in character that Birtwell refuses to be drawn into any detailed discussion of the move – she has remained the soul of discretion for years, after all, and despite the voracious media interest surrounding her late husband's life and death, has kept her private life, past and present, just that. Instead, she says magnanimously: "No one has asked me to be involved, and I will keep saying the same old thing, which is, if it's great, it's very nice to put Ossie's name forward. I've done it in my way and I think he would have been proud of me, and if they do it beautifully, I will be the first to say so."

Celia Birtwell was born in 1941, and grew up in the Manchester suburb of Prestwich with her mother and father and two sisters. "I was there until I was 13, and then my parents moved to Salford 6, on the edges of the East Lancashire Road," she says. Her father, Albert, was an unconventional man. "He was bookish, he loved his books and his garden, and he had three daughters so he was always surrounded by women, poor thing."

Her mother Phyllis, a former seamstress, made all her daughters' clothes. "When there was a party, we'd drag her off to Kendal Milne in Manchester and say, 'I have to have a dress like this', pointing at some sort of Jacques Fath creation on a stand. She made all my clothes from when I was a little girl. I used to sit and watch her, mesmerised, but I never learnt to sew. When Ossie came into my life, he'd talk to her about how you do this and how you do that. They'd have all these private conversations, and I'd be a bit bored really."

Although sewing was clearly never going to be her métier, Birtwell sketched from an early age and, straight from school, in 1956, enrolled at the art school at Salford Technical College. There she met the artist Mo McDermott. "Mo was a very strong character in my life," she says. "He quite changed it. He was an only child and his mother didn't want him to go to art school. He was brought up in Hanky Park, where Coronation Street started. He lived in a little house, you know, back-to-back entries, and had an outside loo where he'd painted a big picture of a cat called Bogey."

It was McDermott who introduced Birtwell both to David Hockney and to her future husband and creative partner, Ossie, for his part already impressing his teachers at art school in Manchester at that time.

Birtwell remembers what Clark was wearing the first time they met. "He had on a V-neck leatherette sweater with a rounded, Victorian collar – this was pre-Beatles but it was a very particular look – and very long winkle-pickers," she says. Birtwell, meanwhile, was far from the shrinking violet in her own choice of attire. "God knows who I thought I was or what I was thinking," she says. "It's too dreadful to think of. One of David's favourite stories is that my father was so embarrassed by how I looked that he'd always walk a few paces behind me in the street. I think I thought I was a mixture of Brigitte Bardot and – you'll laugh when you hear this – Audrey Hepburn! Not very me at all. I used to wear a white piqué Peter Pan collar at college."

They must have made a fine couple: Ossie with his thick, dark hair and fine, biblical features, and Celia all blond ringlets, pretty rounded body and face straight out of a Botticelli painting. With Hockney and McDermott, they went on to form what is still described as a Northern invasion of Swinging London, and it wasn't long before they were its epicentre. Birtwell and Clark lived in west London. He was the most fashionable designer in the world, she the most gifted textile designer of her generation. She moved out of her flat in Ladbroke Grove and into his in neighbouring Westbourne Grove. "I was bitten by cat fleas and he didn't want me to stay there any more."

Ossie Clark dressed the Sixties, the snake-hipped, fine-limbed likes of Mick and Bianca Jagger, Marianne Faithfull, The Beatles, Talitha Getty, Eric Clapton, Britt Ekland... The list goes on. His shows – staged everywhere from the fashionably down-at-heel Dingwalls in Camden to the impossibly grand Royal Albert Hall in South Kensington, and featuring extraordinary models including Patti Boyd, Kari-Ann Jagger, Amanda Lear, and more – were the hot ticket on the international fashion scene.

In particular, the play between Clark's ruffled, draped, bias-cut chiffon and Birtwell's blithe prints, which took their inspiration from romantic places such as Vita Sackville-West's garden and Leon Bakst's costumes for the Ballets Russes, captured the spirit of that moment entirely. Clark's masterful cutting skills might have been more immediately obvious when directed towards a snakeskin maxi-coat, fringed leather jacket or little black crêpe-de-Chine dress, but even the plainest garment bore a label that read: Design by Ossie Clark; print by Celia Birtwell. "I always laugh when I see that," Birtwell says today.

In the early Seventies, with Ossie Clark and Celia Birtwell's celebrity at its height, Hockney painted the portrait Mr and Mrs Clark and Percy. Birtwell was pregnant with her second son, George, at the time. (The couple's eldest son, Albert, was born in 1969.) The picture remains among the most visited works in Tate Britain, and is the best-selling postcard in the gallery's shop to this day. It is a measure of Birtwell's playfulness that it was only after it had been named that she chose to reveal to her great friend that the cat in question is not, in fact, Percy. Instead, she is the far more interesting and aesthetically pleasing Blanche. Hockney responded in kind, telling the Today programme in 1995: "When she told me that, I told her, well, shut up, because 'Mr and Mrs Clark and Blanche' doesn't sound as good as 'Mr and Mrs Clark and Percy'!"

"I don't remember the dress either," says Birtwell today, for the time being at least, having the last word. "I know I was wearing a kaftan when we first posed for the picture, but it's a very good dress, isn't it. I can remember little details of the sitting, probably the problems of it – Ossie's feet were very difficult to draw and that's why they're partially buried in the carpet."

In art-historical circles, much is made of the fact that the work is an inversion of traditional 18th- century portraiture, in as much as it is the man who is seated and apparently passive, and the woman who stands, hands on hips before him, dominating the scene. Birtwell is having none of it. "I've just always stood with my hands on my hips," she says.

The extraordinary role that Birtwell played in the lives of the men who surrounded her is finely captured in Jack Hazan's 1973 film, A Bigger Splash. Part Warholian navel-gazing, part portrait of an artist, the work is remarkable not least for the fact that Birtwell is so clearly the lynchpin around which everything else revolves. There are men queuing up at her door with flowers at every opportunity, in search of everything from a cup of tea and a moan about a love interest to a haircut: Birtwell, it is revealed, among her many other virtues, is also the woman responsible for Hockney's trademark peroxide-blond mop. With babies always in the background, it is clear that, in this permissive milieu, Birtwell represented not only creativity, beauty and wit, but also stability.

In 1974, Ossie Clark may have been, by his own admission, "as famous as egg foo yung", but his private life had broken down. He had split from Birtwell the year before, and Birtwell left the fashion industry to raise their two sons. Ossie fell prey to a self-destructive streak that wasn't to end until his death in 1996.

Even today, Birtwell still describes Clark's instinctive technical ability with wide eyes. "We were all so innocent then, and he was a master," she says. "A master to watch. Hockney always talks about him making a glove in front of him. He made me dresses from start to finish. I remember a lovely lace dress once where I think you could just see the crease in my bottom. Times have changed." She is not alone in her admiration of the designer – his vintage pieces are rare and sell for a small fortune to connoisseurs worldwide.

In 1984, with the help of David Hockney, Birtwell began to work again, setting up a business designing prints for interiors and opening up a little shop across the road from her home. Often taken from 17th-century designs that she researches at the V&A (the museum is currently archiving her work), these have found their way into many a prestigious hotel – Claridge's and the Lanesborough to name just two– as well as discerning private homes. "We couldn't afford marketing," she says. "We didn't have a PR. I always thought, in a kind of way, someone might find me."

Then, in 2002, Birtwell designed the first of four print collections for the French label Cacharel, a deal that was by no means perfect but that led her back to designing textiles destined to be worn. "The fashion industry is more vicious than the interiors one, and, for the home, I can draw my little animals, but I did miss designing for the body," she says.

It wasn't until the deal was signed with Topshop in 2006, however, that things really took off. "The lovely Celia," says Caren Downie, buying director for the store and the woman who has worked with Birtwell most closely throughout. "She's incredibly inspiring and has an awful lot to teach us all. Obviously, the prints are amazing. They're just so beautiful. Also, dresses designed by Celia Birtwell and Ossie Clark are collectible, so they're hardly available anywhere apart from a few vintage ones that are astronomically expensive. This has made her more accessible."

The range comprises vintage pieces reworked, some old prints and some new ones. Garment shapes have been altered predominantly to suit the more casual looks that characterise contemporary dress. It is a formula that has worked and, what's more, has introduced new customers to the store, not least "all Celia's friends", says Downie.

Birtwell confirms that many of these have been back in touch since her new-found success, but the lady herself is more interested in investing the proceeds and sharing them with her family and her partner of 21 years, Andrew Palmer, than in socialising. "People think I'm strong," she says. "But I couldn't have done it without my friend Andrew Palmer. I've got six grandchildren, you know, and I've always wanted a cottage in the country. Maybe I'll get one now."

In the end, Birtwell is just as charming, modest and direct as her work, "which I like to think it's quite fresh and joyous", she says. "The fact that it has a life and it has gone on means that even I can see that it has a look of its own, whether you like it or not. It's quite distinctive and, I think, quite honest. Hopefully, it also has a lot of love in it, because I have always loved doing it."