Lee Miller was on the cover of American Vogue at 20, became part of the surrealist art movement in the 1930s and would go on to be the only female combat correspondent for the allies during the Second World War. Yet the photographer described as “one of the most remarkable female icons of the 20th century” was thought to have left just a few hundred works when she died 35 years ago.

That was until her son, Antony Penrose, unearthed a treasure trove of negatives in the loft of the family home. Thousands of never-seen-before images will shortly be made available online.

Mr Penrose told The Independent: “No one knew about these photographs, and that was the way she wanted it. She deliberately buried them.”

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Finding the boxes in the attic of Farley Farm House was a “cathartic moment” after a troubled relationship, he said, and he hopes they will help bring his mother’s work to a wider audience.

“She is not recognised enough, but we are working on it and making huge progress. It is deeply satisfying to notice how relevant she is to young people,” he said.

The 3,000 images, available online from 23 April on what would have been Miller’s 106th birthday, are the first instalment in a process of making all 60,000 negatives discovered freely available on the internet.

While Mr Penrose discovered the boxes in 1977, the family only began the painstaking process of transferring the material onto a digital format three years ago. They hope to have all of the images widely available within five years.

These negatives include shots of the Alsace campaign during the Second World War, the liberation of Paris and harrowing images of starving children in Vienna after the end of the conflict.

They also include unseen shots of writers and artists from her social circle from Dylan Thomas and Henry Moore, to Jean Cocteau, Colette and Pablo Picasso.

Mr Penrose said: “She mentioned nothing of her career or her achievements. She hid all of that and deflected anybody from discussing it. She was very modest.” Their home was packed with modern art, but only had two of her photographs; she rarely documented the family, he added.

Miller was born in Poughkeepsie, New York and would be on the cover of American Vogue in 1927.

Several years later she sought out surrealist photographer Man Ray in Paris, becoming his assistant and then his muse and lover.

The outbreak of the Second World drove her to embark on a career in photojournalism, and she documented subjects from the Blitz to the liberation of the Buchenwald and Dachau concentration camps. Mr Penrose said: “Nobody would give her a gun; the camera was her weapon of choice.”

Yet the end of the war left her embittered. “She had such high hopes for the outcome of the war, that she was really crushed when she found peace had not brought the freedom and justice she had wanted,” Mr Penrose said.

She suffered from post-traumatic stress disorder as a result of what she had seen in the war, and also from the sexual abuse she had suffered as a child.

Mr Penrose said their relationship was “fraught” as she battled with the disorder and alcoholism. “Towards the end of her life, she recovered, which was a magnificent achievement. We had a rapprochement towards the end of her life.”

After the war, Farley Farm House where she lived with Antony’s father Roland in 1949 – and which now operates as a museum about their work – entertained prominent artists and writers of the day.

Mr Penrose said he had been particularly fond of Man Ray and Picasso, when they visited. “I was instantly aware Picasso was a remarkable person. He was magnetic he had an extraordinary charismatic quality.”

Since the retrospective at the Victoria & Albert Museum in 2008, “interest in her work began to take off,” Mr Penrose said.

The core part of her audience is women between 18 and 25. He said: “I’ve met a great many women who tell me what an important role model she was for them. How they never would have broken out and done their careers if they hadn’t read what she had done. She would have loved it.”