The unseen Lee Miller: Lost images of the supermodel-turned-war photographer go on show

Thousands of pictures of the Miller - as well as her own shots of friends from Picasso to Dylan Thomas - had been lost to the world, hidden in an attic. But now they are about to be made available to the public for the very first time...

Lee Miller was a shape-shifter: a beautiful model with a brilliant artistic eye of her own, a fearless photojournalist turned unconventional homemaker. She was a high-fashion fixture in the 1920s, discovered when the publisher Condé Nast pulled her out of the way of oncoming traffic – and then on to the cover of Vogue.

She moved to Paris and became a Surrealist, not only a muse to Man Ray, but a pupil, learning photography for herself. She became a war correspondent, documenting the Second World War's front line and concentration camps. She then settled in Sussex with her husband, the British Surrealist Roland Penrose, in 1947, but continued to play host to, and photograph, artistic giants of the 20th century: Pablo Picasso, Max Ernst, Joan Miro, Henry Moore.

She also, according to her son Antony Penrose, became a very difficult person to be around, and a poor mother. Suffering post-traumatic stress disorder after what she had witnessed during the war, Miller turned to drink. "For 20 years of my life she was an alcoholic, and a depressive, and that's difficult for a kid. We became really embattled and we fought like crazy," he says, with simple candour.

Miller died in 1977, a minor figure in the art world. Until, that is, Penrose stumbled across a vast tranche of her work, hidden in the attic, which would cement her reputation as a pioneering female photographer and form the basis of the Lee Miller Archive at her home, Farley Farm House. "My late wife wanted to find a picture of me as a baby – we had just had our first daughter and she wanted to see whose nose our baby had got," Penrose begins. "While she was searching, she found this huge pile of cardboard boxes, which contained this incredible treasury of about 60,000 negatives, and letters and maps and that kind of thing."

Miller had turned her back on her own work towards the end of her life, wanting to "bury it", Penrose suggests – although he has a hunch that she did, in her heart of hearts, "appreciate how important it is historically and artistically". And so it has proven: there have been major Miller exhibitions around the world, and much has been written about her dramatic life and impressive oeuvre. Indeed, the interest is such that this private, family-run collection could not cope – too many academics and fans want to root around. So the Lee Miller Archive is going online.

The first batch of photos, some 3,000, have been converted to digital images and will be made available on a new website from Tuesday – Miller's birthday; a further 5,000 to 7,000 pieces will be added each year thereafter until the entire haul is digitised.

"The launch [includes] a broad selection of portraits of artists, quite a lot that have never been seen before. There are also war images that have never been seen before," says Penrose. Featuring subjects from Henry Moore to Clark Gable, Jean Cocteau to Fred Astaire, the images reflect the scope of both Miller's journalistic assignments, and her glamorous, bohemian circle. Several of these pictures are being published for the first time on these pages.

Miller's portraits often have an emotional directness or an easy breeziness that reflects the friendships she documented – but her shot compositions also frequently favour the eccentric, or have what the Surrealist André Breton termed "convulsive beauty", with unlikely juxtapositions or surprising angles that raise a smile.

So what is the defining quality of her work, and how did she coax such perceptive portraits out of such starry figures? "Whether she was photographing fashion or the war, first and foremost she was a Surrealist," says her son. "That quirky way of looking at things, seeing behind the picture, the joke, the pun – it's all part of her lexicon."

He hopes that not only will the online archive be useful for researchers – who often ask after the same images again and again, when " in most cases there are several alternatives that would be more fun to use because they haven't been out of the box" – but that it will also help new people discover her work. And there will be many images of Miller herself, too, taken by others, including her father Theodore (a keen amateur photographer), her friends and lovers (of whom there were many), and her family.

Penrose suggests some will reveal new sides to her: "Pictures that [my father] Roland Penrose or that I took put her in a different context – there she is, in her apron in the kitchen, stirring a pot. People don't think of her like that; they expect to see her with a helmet on, storming across Europe."

Finding the archive has completely changed Penrose's life (he was formerly a farmer), although when they initially discovered those dusty boxes after Miller's death, he says, "We hadn't a clue what we'd got." His life since has been dedicated to promoting his mother's varied career, and he has written books, plays, articles and lectures about her, his father, their work and their artistic coterie.

Although he was brought up in a house stuffed with famous pieces of modern art, skittering around the heels of Picasso or chatting with Miro, for Penrose it all felt normal: "They were all so friendly and funny. My parents never made a fuss about it – they weren't celebrity hunters." As well as not realising quite how significant such characters were, he also underestimated Miller: "When she died I had a very hazy perception of her achievements."

Documenting Miller's work has, not surprisingly, totally changed his understanding of her as a person. "I feel very good about her now, having felt very hostile towards her," explains Penrose. "It led me to have a really profound re-evaluation. People who knew her before the war knew a very different person, this adventurous and articulate and highly daring person. In the post-war years, stress really got the better of her – but she shook it off, and that was, I think, her finest achievement. Towards the end of her life, thanks again to the intercession of my first wife, we became friends. But I didn't know her."

Not that the process of discovery was always easy. As the most famous female combat photographer during the Second World War in Europe, Miller followed the US army to Normandy shortly after D-Day, witnessed the Liberation of Paris and documented the concentration camps of Buchenwald and Dachau. When I ask Penrose if looking through such images was harrowing, he replies, "Oh, god, yes."

While the holocaust images were, of course, horrifying, the one that really still haunts him is of an 18-year-old German soldier. "He's died horribly, both his hands are blown up, and on the back she has written: 'This is a good German – he is dead.' That shocked me to the core, that this person, who despite all her troubles had still been tremendously humanitarian, could write that about this 18-year-old boy. That more than anything made me realise how deeply held her hatred for the Germans was in that moment."

But he is also emphatic that Miller, as a rare example of a female front-line photojournalist, was bringing a different perspective to some of the greatest horrors of the century. "We have arguments about this a lot: do women see, and photograph, extreme things such as combat differently to men? The short answer is yes, definitely. It's wrong to generalise, but women tend to have a more humanitarian, and more visceral, eye. They enter into a more emotional connection with the subject. It was unusual then; it's still unusual now, even though there are a great many extraordinary women photographers out there doing terrifying things – but Lee was one of the first."

A striking feature of the modern audience for Miller's work, wherever it is exhibited today, is that it always includes an unusually high proportion of young female visitors aged 18 to 25. "I find it very touching that she's still relevant to young people, and that there are a great many young women who regard her as a role model," says Penrose, who is quite convinced Miller would love it too.

"It makes you think, doesn't it, what can be achieved – particularly in those days, when the advantages were so stacked against her," he says. "The first part of her life she was what you would call a supermodel, for Vogue and Vanity Fair, being photographed by [Edward] Steichen and Man Ray and looking absolutely incredible. And then, not many years later, she is this person indistinguishable from the GIs, in combat gear and a helmet in a pile of rubble – how great a contrast can you get?"

The new website, leemiller.co.uk, goes live on Tuesday

© Roland Penrose Estate, England 2013. The Penrose Collection. All rights reserved. © Lee Miller Archives, England 2012. All rights reserved