The prime of Miss Jean Muir

IF JEAN MUIR had not existed, it would have been necessary for Muriel Spark to invent her. And perhaps she did. For, like her namesake Jean Brodie, the illustrious designer and doyenne of British fashion did not suffer fools gladly, if at all. Her perfectionist standards, ceaseless energy and relentless application certainly made her as formidable a role model for the fashion world as the Edinburgh schoolmistress was for her pupils.

Indeed, it was impossible to meet Miss Muir - she was Jean only to those closest to her - and not be reminded of Miss Brodie. Yet you sensed that if Muir, with her wicked sense of humour, had ever met the fictional character she would have put her firmly in her place, for she was a mistress of the put-down.

Nonetheless, like Spark's ruthless heroine, Muir had a mysteriously complicated private life. Speculation was rife that she also led a secret existence, despite the fact that she was married for 40 years to the German-born former actor Harry Leuckert, with whom she co-founded their company and shared homes in London and the Borders. He has described their relationship as "wonderful and loving, but never singular" - he has a daughter, Friederike, with another woman.

Muir died of breast cancer in May 1995, aged 66, but Leuckert - who later married Friederike's mother Ingrid - remains involved in the family business. Friederike, now 30, who manages the flagship Jean Muir store in London, has said that her existence wasn't a secret, although her father continued to live with Muir while paying regular visits to her in Germany. "People find that hard to believe, but Miss Muir knew about it, my mother knew about it and I knew about it. Nothing was hidden, but nor was it public," she says.

It's not surprising that this was an unconventional marriage, as there was nothing conventional about Muir. In the workplace, for example, not only did Muir's staff need to comply in the normal professional sense, but she also appraised potential workroom candidates for the neatness of their presentation and handwriting. Good grooming was a prerequisite, recalls Sinty Stemp, author of Jean Muir: Beyond Fashion. This beautifully illustrated book, a celebration of the private life and extraordinary vision of a woman regarded as one of Britain's top designers, demonstrates why her sinuous vintage designs are still being worn and treasured today by women such as Tilda Swinton, Sienna Miller, Kate Moss and Stella McCartney.

The publication marks the 40th birthday of Muir's eponymous label, which she launched in the middle of the Swinging Sixties to rave reviews, winning a following of 'Muirettes' that included Princess Alexandra, Lauren Bacall, Diana Rigg, Judi Dench, Barbra Streisand, Elisabeth Frink and Maggie Smith. Most famously, though, it is Muir's one-time house model, Joanna Lumley, whose name is most frequently associated with the designer's work. "I didn't go to university. I went to work for Jean Muir instead," says the actress who began modelling for the designer's original Jane & Jane label in 1964.

Working for Muir could be daunting, acknowledges Stemp, who is sales director for Jean Muir today. Many feared the brand would not survive its creator's death, but it continues to flourish from its all-white store in London's Conduit Street, which opened in 2004.

The continuing success of Jean Muir, which is worn by Charlotte Rampling, Julie Walters, Juliet Stevenson and Vogue editor-in-chief Alexandra Shulman, is due partly to the loyalty of the designer's staff, but also to the management skills of Friederike and her husband Nicolas Steineke, who never interfere in the creative process. They never met Muir, but have the greatest respect for both her work and her philosophy. "I've learnt so much that in a way I do feel I knew her," Friederike says.

Stemp, who has been with the company for 20 years, did know her, spending seven years as the designer's personal assistant. Her book therefore offers a unique portrait of the woman in all her puzzling, paradoxical prime, though she does admit that Muir remains enigmatic - "even to me".

She was naturally shy, but Muir's intensity and refusal to compromise was sometimes mistaken for grandeur. "She was grand, but rather in the sense of being eminent. For all of us who worked with her, she was a remarkable renaissance woman who merged the fields of design, craft, art, education and politics in her career," writes Stemp.

"Although a diminutive figure, she was a towering presence and an authority in her field - she never lost her creative spark. Her petite form masked an unflinching determination as well as a witty sense of humour. Expecting the best at all times, she inspired enormous respect and loyalty in her friends, customers and staff."

Muir, invariably dressed from head to toe in navy-blue or a little black dress, had "an altruistic crusading zeal for her industry, its necessary skills and its nascent talent", according to Stemp. She also loved pink champagne, Man Ray, Danny La Rue, Mozart and the Victoria & Albert Museum, of which she was a trustee for a decade.

Above all, though, Muir, the daughter of an Aberdonian father and an English mother, passionately loved Scotland and the Scots, cheerfully boasting, "I'm half!" Leuckert acknowledged this powerful affinity earlier this year when he gifted her extensive and unique archive to the National Museum of Scotland. The collection comprises 18,000 individual items, including patterns, toiles, accessories and finished garments, and now a major Jean Muir exhibition is planned by the museum for 2008.

BORN in London on July 17, 1928, Muir was the daughter of Cyril Muir, a draper's floor superintendent, and his wife Phyllis Coy. She attributed her creative pragmatism and legendary self-discipline to the Scottish ancestry on her father's side.

Muir was always reticent about her early life, but her parents parted while she was still young and she and her brother Christopher moved from London to Bedford, to the home of her mother's family, soon after she reached school age. Academically unimpressive, she showed a precocious talent for needlework, claiming to have been able to sew, knit and embroider by the age of six, and to have completed her first skirt at the age of 11.

At Dame Alice Harper girls' school in Bedford, she came under the tuition of her own Miss Brodie - an art teacher who had a profound influence on her. "I can remember from the age of ten knowing the great painters and their most famous works," she once said. It was this visual literacy that Muir would take with her and advance throughout her career.

After several short-term jobs, she found work at London's Liberty store, graduating to designing for the new Young Liberty department. It was, she said, her spiritual home, the art school she never attended. She then moved to Jaeger, where she spent six years creating dresses, sweaters, coats and suits and learning the complex disciplines of seasonal production.

Under her own label, Jane & Jane, Muir began dressmaking in 1962, backed by the fashion house Susan Small. When others were still transfixed by youth, Muir had already formulated her vision - the look that made her famous and ensured she survived the 1960s. "Engineering with fabric," she called it.

In 1966, Leuckert borrowed the capital to set up Jean Muir Limited and the designer focused on luxurious materials and simple shapes; liquid silks, clinging jerseys, soft suedes and satiny tweeds. All bear the Muir handwriting, her special way of cutting, stitching and seaming.

The designer's designer, Muir was revered by the likes of Jean Paul Gaultier, Issey Miyake and Giorgio Armani. The late Franco Moschino bowed down in front of her when they met in a nightclub. And fashion pundits today point out that the essence of her style can be seen everywhere, from the rigorous tailoring of Preen and Sinha-Stanic to the top-stitched jerseys, crpes, wools and suedes of Azzaro, Chlo, Fendi, French Connection and Mulberry.

Yet Muir detested the word 'fashion'. "Don't call me a fashion designer - a self-important, pretentious term," she told Vogue in 1978. "Just do it importantly." She believed in the artist-craftsman, with technical training being the only serious basis for a designer - concentrating on "more craft and less art". Ultimately, her focus was on style. Fashions come and go, but style goes on for ever.

Muir herself was as stylish as they come - in her look, her language and the way she moved in her chosen surroundings. Her stunning all-white flat, with Venetian-glass furniture in the bedroom, was a typically stylish lifestyle statement.

Muir's twice-yearly fashion shows were enormous fun. Held in her chic Mayfair showroom, they were like informal cocktail parties. Her friend Bobby Short would play jazz as champagne was served to potential buyers seated on little gold chairs beneath glittering chandeliers. The models were her favourites - India Hicks, Veronica Webb, Anna Pavlowski and Joanna Lumley - "girls who don't look like models".

It was characteristic of Muir that she kept her terminal illness secret even from close friends, working right up to the end. Such was her life-enhancing courage and determination that she had begun fundraising for the National Museum of Scotland project in Edinburgh shortly before her death. Her husband and Jean Muir Limited carried on this work, pledging money in her memory, as did many friends and loyal customers. The company is named on the founder's stone at the entrance of the award-winning National Museum of Scotland building, while the museum's Silver Room is dedicated to Jean Muir herself.

Years ago, I went to a glitzy fashion event in London dressed in a printed blue silk Jane & Jane dress that was easily 20 years old, as my mother had bought it and worn it in the early 1960s. The American designer Ralph Lauren dashed up to me and exclaimed, "You're wearing Jean Muir." But it's only her old Jane & Jane label, I replied, blushing to have been outed in my mother's frock. "Only!" Lauren exploded, rolling his eyes and throwing his hands up in the air in despair. "It is simply exquisite," he said, closely examining the cut. "Very Jean Muir!"

Now I realise that this little dress - Joanna Lumley still owns one, which was shown in the exhibition From the Bomb to the Beatles - had something beyond fashion, something the fashion writer and lecturer Colin McDowell recently termed "the svelte and logical perfection of a Jean Muir". It would be worth several hundred pounds, had it not been consigned to Oxfam a decade ago. How Muir would have giggled!

• Jean Muir: Beyond Fashion by Sinty Stemp (Antique Collectors' Club, 25). Published Novermber 29.