It includes a harrowing portrait of despair by Edvard Munch; a laundry girl shuffling down a Paris street by Pierre Bonnard; and people promenading through the Tuileries gardens by Édouard Vuillard.

The images are contained in one of the most important portfolios of prints ever produced – something which, it had been assumed until now, did not exist as a complete set.

Sotheby’s has announced that the only known complete set of a portfolio titled Les Peintures-Graveurs, published by one of art history’s most important dealers, Ambroise Vollard, had indeed come to light and will be sold at an auction in London next month.

Félix Vallotton’s Le Premier Janvier. Photograph: Ambroise Vollard Portfolio

In the world of prints, things don’t get any bigger than a discovery like this. Séverine Nackers, the head of prints in London, said it was “astounding” and even more unusual because the initial approach was a speculative one. “It was a random inquiry which, with the internet, is so rare these days.”

The anonymous European seller sent Nackers’ colleague Yessica Marks an image of the most famous print in the portfolio, Munch’s Angst. When it became clear there were more, excitement rose.

“We couldn’t believe it,” Marks said. “The next morning we jumped on a plane and drove for five hours to this small town and we were so excited to see the portfolio in person.”

Pierre Bonnard’s La Petite Blanchisseuse. Photograph: Ambroise Vollard Portfolio

The portfolio was created in Paris in 1896 by Vollard when he was 30. Initially produced in an edition of 100, it contains 22 prints by artists who also include Auguste Renoir, Odilon Redon, Suzanne Valadon and Félix Vallotton, whose paintings and prints are currently on display at an exhibition at the Royal Academy of Arts.

Vollard struggled to sell the portfolio in its entirety so he often broke them up and sold the prints individually, which is why a complete portfolio is so rare. None has ever come to market and, as far as the auction house is aware, no complete portfolio exists in any museum collection.

It is thought the newly discovered portfolio was purchased in Paris in 1920 and was passed down through the same family with the current owner unaware of its rarity.

Because it has never been sold before, Sotheby’s has put on it a strikingly wide estimate of between £500,000 and £1m.

That may be conservative given that a print of Munch’s Angst has sold for more than £500,000 at auction as a single lot.

Angst is arguably the star of the portfolio. It was made shortly after Munch’s arrival in Paris, encouraged there by the city’s thriving print activity and much better reviews for The Scream than he was getting in his native Norway.

Produced in collaboration with the eminent printmaker Auguste Clot, the image is the first colour lithograph ever produced by Munch.

Edvard Munch’s Le Soir (Angst). Photograph: Ambroise Vollard Portfolio

He went on to become one of the greatest printmakers, a medium which allowed him to experiment more radically and freely with form and meant his works could be seen by a mass audience.

Angst, or Le Soir, is considered a variant on The Scream and shows a funeral procession against a background of a turbulent, terrifying red sky. If you want a night in contemplating loneliness, alienation and loss this is the image to have.

Nackers said it was only the second portfolio published by the young Vollard, “who elevated lithography, and the work of many radical artists from across Europe, to international acclaim”.

“Vollard showed astonishing foresight,” she said. “And the portfolio stands testament to the work of one of history’s greatest publishers, and to the history of printmaking as a whole.”

The portfolio will be sold in Sotheby’s prints sale in London on 17 September.