Previously unpublished portraits of the composer Igor Stravinsky are among hundreds of drawings of musicians and other leading cultural figures of the last century that have come to light following the death of the artist Milein Cosman.

In a cache of drawings that offers an unparalleled glimpse into the rich musical culture of 20th-century Britain, more than 1,300 drawings of 220 musicians have been given to the museum of the Royal College of Music (RCM) in London.

Cosman, who died last year aged 96, sketched composers including Benjamin Britten, Richard Strauss and Leonard Bernstein and the cellist Mstislav Rostropovich. She drew them from life, observing them during rehearsals or concerts, often when they were unaware of her presence.

In the collection given to the RCM there are 50 drawings of Stravinsky alone, most sketched at the BBC Studios in Maida Vale, west London, when the composer visited London in 1959. Julian Hogg, one of Cosman’s close friends, said: “She was there for three days and she drew all the time.”

Igor Stravinsky. Illustration: Milein Cosman. © The Cosman Keller Art and Music Trust / Royal College of Music

He remembers that she never left home without a sketchbook: “It was a complete obsession. She just had to draw all the time.” But, as her executor, even Hogg has been taken aback by the sheer number of drawings among her possessions.

“There are literally hundreds of sketchbooks which we haven’t looked through yet,” he said last week, standing in Cosman’s home in Hampstead, north-west London, where huge boxes bulge with sketchbooks.

Hogg, the chairman of the Cosman Keller Art and Music Trust, found a sketchbook with yet more images of Stravinsky: “That’s never been published, certainly.”

Another folder featured the composer Peter Maxwell Davies, hunched over a musical score, and Vaughan Williams at a piano, seen from the back. There, too, was pianist Vladimir Horowitz, and violinist Ida Haendel: “In the second half of the last century, there were hardly any musicians who she didn’t draw.” Cosman came to Britain from Nazi Germany in 1939 and spent most of her life in the UK, marrying the broadcaster and musicologist Hans Keller. She studied at the Slade School of Art and earned a living drawing for various publications. With sparse lines in ink, pencil and conté crayons, she captured movement and emotion, while achieving a precise likeness. Musicians were often depicted in different poses drawn one after another, building a “narrative” capturing how they moved and gestured.

The art historian Sir Ernst Gombrich once predicted: “Posterity will be grateful to Milein Cosman above all for the sureness of her eye, with which she has succeeded in capturing the unique quality of so many of our distinguished contemporaries.” Professor Gabriele Rossi Rognoni, curator of the RCM Museum, believes that most of Cosman’s drawings have never been seen before.

Richard Strauss. Illustration: Milein Cosman. © The Cosman Keller Art and Music Trust / Royal College of Music

The museum was told by Cosman to expect a bequest, but did not know the scale of her donation. “We know she was extremely fast at drawing, and didn’t look at the page while she was drawing. She was capturing immediately what she saw,” said Rognoni. “I like the ones where the personality of the person really comes out, like the Rostropovich one, because you have the whole body around the instrument – [which] he’s embracing.”

On 20 February, the RCM will stage a concert and exhibition, titled The Gift of Music: Milein Cosman Collection. The programme will feature works by some of the musicians featured in the sketches.

Financial support from the Pilgrim Trust will enable the full digitisation of the collection, making it available to the public and an invaluable resource for musicians and historians. Many of Cosman’s portraits relate to RCM alumni. She claimed, however, that, “What I really like to draw best are people who work: fishermen and road menders. Yes, it must be said, that really a lot of people have quite wonderful faces”.