She was adored by the Paris fashion industry and could count royalty and stars among her clients. But the life of one our most popular designers was shrouded in mystery, reports Gillian Bowditch It is a paradox so entertaining it could stage its own cabaret. Jean Muir – one of Britain's most important 20th-century fashion designers, whose company celebrates its 40th anniversary this year – built her career from the antithesis of fashion. Everything about her, even the clothes she wore, seemed to militate against her career.

"Miss Muir didn't like to be thought of as a fashion designer," says Joanna Lumley, who became one of Muir's house models in 1964. "She saw herself as a dressmaker. She was an incredible stickler for accuracy. If there was a piece of stitching slightly off, out it would come. We all adored her, but we were also quite afraid of her."

With her trademark Louise Brooks bob, chalky complexion, damson lipstick and unrelenting navy uniform, Muir used her dramatic personal style as a mask behind which she hid an intensely private and sometimes painful life.

Although she died 11 years ago at the age of 66, her international renown – the French knew her as "la reine de la robe" – lives on. Miles of column inches have been devoted to her understated but exquisite tailoring, but the details of her life are sketchier than the drawings that heralded each new collection. No biography has been written of her.

A vision as focused as a high-precision telescope runs, thread-like, through the clothes. "Evolution, not revolution" was her philosophy. But Muir's life is an intriguing storyboard of contradictions.

Now one of the most beguiling – her claim to be Scottish – has secured her remarkable archive for the nation. Donated to the National Museums of Scotland by Harry Leuckert, her husband and business partner, the Jean Muir Archive runs to 18,000 individual items, including patterns, toiles, accessories and finished garments. It is believed to be the largest and most complete archive of a fashion designer in a museum collection anywhere in the world and could have fetched hundreds of thousands of pounds at auction.

As a resource for designers and students, it is priceless and a coup for NMS, not least because Muir was a trustee of the Victoria and Albert Museum in London.

"It's not just the scale of the collection, but also the fact it shows the entire working process," says Fiona Anderson, curator of textiles at NMS, which is planning a big Muir exhibition in 2008. "That is so appropriate, because she was known for her technical skills. Her work wasn't just some airy-fairy notion of creativity. It is an incredibly valuable resource and is internationally significant. We are thrilled to have it."

In the offices at the top of the museum, Anderson has laid out a series of Muir's sketches, patterns, toiles and accessories for The Australia Collection, designed to celebrate that nation's bicentennial in 1988. A riotous blue, grey and yellow felt jacket decorated with brooches hangs beside more restrained dresses of the kind for which Muir was renowned.

"Jean Muir was a founder member of the National Museums of Scotland and involved with the fundraising process," says Anderson. "But more fundamentally, although you have to go back several generations to get to her Scottish roots, she felt her cultural identity very strongly."

"She had a great passion for Scotland," says Lumley, who remained a close friend and was one of only two clients for whom Muir did private fittings. (The other was Princess Alexandra.) "She loved the tweed industry and the knitwear. She couldn't bear dilettantes and identified with Scottish craftsmen. I remember her coming back from Scotland with armfuls of beautiful, lacy knitwear, which she adored."

Muir was known by some as the "Scottish Chanel", but she could just as easily have been fashion's Miss Jean Brodie. There was something Sparkian about her voracious appetite for books, her scholarly mind and sense of theatre – she was a blue stocking, literally and metaphorically. She was also a perfectionist who revelled in her reputation as the crème de la crème of dressmakers. She identified with the Protestant work ethic, but her Calvinism was combined with a romantic notion of the Scots as an artistic race.

"She often talked about her Celtic blood and was proud of the fact she wasn't an Anglo-Saxon," says Sinty Stemp, head of marketing and press for Jean Muir. "She felt she had been born with a very creative eye and that the Celts were an artistic race. She attributed her steely drive and independence to her Scottish roots."

In fact Muir, who was self-taught, was born in London in 1928 and went to a girls boarding school in Bedford, where she learned to sew. Her first job in textiles was with Liberty as a stockroom assistant, but her artistic eye and nimble fingers meant she was quickly promoted. Before setting up the now defunct label, Jane and Jane, part of the Susan Small Group, she designed for Jaeger. In 1966, with Leuckert's backing, Jean Muir Ltd was founded.

You have to go back to her great-grandparents to find the link with Scotland. Her paternal great-grandfather, RC Muir of Elderslie, was an engineer and "pioneer of the frozen meat trade". He left Scotland for London in 1887.

But just as Muir's notion of nationality had little to do with borders, so her idea of fashion had nothing to do with trends. She used the word as a verb, not a noun.

"She was aware of trends, but they didn't deflect from her style," says Stemp. "She brought the discipline of the couturier's atelier to ready-to-wear. Her clothes had great integrity. They are very feminine, but not aggressive or cloying. They move beautifully on the body. Her signature look is quickly identified, but it never shrieks you are wearing 'a label'."

Lumley still has a large suitcase of Muir's designs, many with the handmade silver or Perspex buttons commissioned from young British craftsmen that were a feature of her work. "She loved the deliciousness of beautiful things," says Lumley. "I remember working for her in the 1960s on the fourth floor of her studio in Great Portland Street and she was just enchanted by this beautiful Indian printed silk. I still have a little Jane and Jane dress made of Liberty print silk. Those clothes are very difficult to come by now and that particular dress ended up in the Imperial War Museum in an exhibition entitled From the Bomb to the Beatles."

Muir's longevity as a designer at the top of her game for more than 30 years is unrivalled in the British fashion industry. Along with Biba and Mary Quant, her clothes defined the 1960s. When Sienna Miller needed to exude Sixties' sophistication in the remake of the film Alfie, it was to Muir she turned. Vintage Muir now sells for hundreds of pounds.

"Skilful cutting was something she was known for," says Anderson. "If you look closely at a very simple little black dress, you see all these amazing details and clever ways of cutting that make it sit beautifully on the body."

Muir inspired loyalty and trepidation in her staff in equal measure. "She was tiny," says Stemp, "but she had an aura. She was always dressed in navy blue and was so neat and fastidious."

Sir Roy Strong, who knew Muir through her work at the V&A, says: "When her passions rose high, her features could assume a striking resemblance to Munch's The Scream."

"Everyone called her Miss Muir," says Lumley. "It was ages before I felt I could call her Jean. She was like Dame Ninette de Valois [who founded the Royal Ballet] in that she inspired natural courtesy. She always dressed in navy blue with navy blue stocking and neat little shoes. But she had an incredible youthfulness. People just adored her."

For women of a certain age and stature, her clothes were a godsend. It was to a Jean Muir dress that Camilla Parker Bowles turned for the raspberry evening gown she wore on the night her engagement to the Prince of Wales was announced.

Lumley recalls salon sales at which actresses and aristocrats would "strip down to their underwear" and rummage through racks of end-of-season bargains. "She was enormously good fun," says the actress, "and she had a very eclectic group of friends. Jazz was always played at her shows. She was the coolest of the cool."

But even to her friends she was a riddle, an enigma wrapped in a navy jersey dress. "I have no idea where she stood politically," says Lumley. "She had a very wide range of interests and tastes. I remember her and Harry taking me to a Danny La Rue show. She was so secure in her style that she never felt threatened by trends."

Muir's all-white flat behind London's Albert Hall, where visitors had to remove their shoes for fear of marking the white rubber flooring, was renowned for its minimalism. Few knew of her bolt hole in Northumberland, which was crammed full of books, colourful rugs and antiques. There she swapped the navy blue uniform for country clothes. "It was the absolute polar opposite of the London flat," says Lumley.

But perhaps the biggest mystery of all was her marriage. Muir married Leuckert, an actor born in Germany, in 1955. They had no children. What is not widely known is that her husband, who remains chairman of the company, had another family in Germany. After Muir's death he married Ingrid, the mother of his daughter, Friederika. She was born in 1976 and although she never met Muir, she now runs the Jean Muir shop in Conduit Street, central London. Her husband, Nicolas Steineke, 29, is the managing director.

"My father-in-law married my wife's mother," says Steineke. "He is also her natural father. My wife and I grew up in Germany until 1996. We didn't know Miss Muir. We came over here after her death."

Friends are in no doubt Leuckert adored Muir and it was an intense partnership. "We loved each other very much. We were very close emotionally and creatively," is the only public statement he has made on the relationship, which Strong described in Muir's obituary as "the perfect marriage".

If Muir knew of Friederika's existence, she didn't speak of it and her friends remain loyal to her wish for privacy. Muir's great love was her work. It was no secret among their close circle that Leuckert, who shunned the limelight, had his own life and interests.

According to Steineke it is Leuckert who is responsible for preserving the remarkable archive and whose foresight meant most of it was in pristine condition.

He also steered the business through financial ups and downs in the 1970s and 1980s, when a stake was sold off to the textile group Coates Patons and then bought back.

"It is a very fickle industry," says Steineke. "Miss Muir built up an extreme loyalty over 40 years. She was not really interested in the whims of fashion. But there were recessions, a number of fairly volatile phases."

Her loyal band of customers saw none of this, however, just collection after collection of enduring, tasteful, unmistakable clothes. That was the Muir "genius".

"Jean Muir was very discreet," says Lumley. "She didn't talk about personal matters. We knew Jean had illnesses and operations, but we didn't know what they were and she never spoke of them. No kind of feebleness was permitted. Her death came as a great shock."

Muir died from breast cancer on May 28, 1995. She had entered hospital two days earlier complaining of feeling unwell. "Only the inner circle knew of it and she stayed at work till the end," says Steineke. "The medical condition had only been diagnosed a fortnight or three weeks earlier and the treatment was unsuccessful. It all happened very quickly."

"Her death was an incredible shock to everybody," says Stemp. "We knew she was having some treatment and then she didn't come back from the treatment." It may be that Muir had some idea of the fragility of her health. In the period before she died she gave her trusted team of designers greater responsibility and freedom.

"We know the Jean Muir signature details and shape and we've had the archive to work from," says Stemp. "She was such a presence."

A decade after her death, Muir lives on in the clothes she fashioned. And because of the foresight and generosity of Leuckert and his secret second family, a century from now her fabulous legacy will live on in the National Museums of Scotland.