Welsh writer Manon Steffan Ros has won the 2023 Yoto Carnegie medal, the UK’s most prestigious children’s books award, the first time a translated book has won in its 87-year history.

Judges described Ros’s young adult novel The Blue Book of Nebo, set in a post-apocalyptic Wales, as both “heartbreaking” and “rich with Welsh heritage”. The work, originally published in Welsh, was translated by Ros herself.

“I’m absolutely delighted,” Ros said. “I used to see the word Carnegie on the cover of my favourite books when I was a child, so this means a great deal to me.”

The medal for illustration went to Jeet Zdung for Saving Sorya: Chang and the Sun Bear, by Vietnamese wildlife conservationist Trang Nguyen. The book, which features manga-inspired illustrations and watercolour scenes, is based on a true story about a young woman working on her own to save a bear. It is the second year in a row that a graphic novel has won the illustration prize, formerly known as the Kate Greenaway medal.

The winners of the medals were announced at a London ceremony on Wednesday hosted by Lauren Child, who won the illustration award in 2000 for her first Charlie and Lola book.

Ros’s novel charts the relationship between a mother and son who struggle to survive in their isolated hilltop home after a nearby nuclear disaster leaves them without electricity or running water. “As this is the first book in translation to win the Carnegie, my hopes are that readers, and particularly publishers, are more open to books in translation,” Ros said.

The chair of judges, librarian Janet Noble, said of Ros’s novel: “The world-building and distinct voices of the two main characters, the son and his mother, are expertly realised and the reader is compelled to question their own relationship with the modern world”.

Ros lives in Tywyn, north Wales, and has written more than 20 books for adults and children. The original Welsh version of The Blue Book of Nebo, Llyfr Glas Nebo, won the 2019 Wales book of the year.

Ros and Zdung both receive a £5,000 cash prize and £500 worth of books to donate to a library of their choice. Ros is planning to donate to Tywyn library, where she wrote some of her books when she didn’t have internet access at home, while Zdung will support libraries that Nguyen has set up near Vietnamese national parks.

Zdung said he hopes Saving Sorya “will offer a deep knowledge of wildlife while impressing upon young people the importance of its conservation”. In preparation to illustrate the book, he conducted field research in Vietnam and Cambodia. “Roaming through the jungle and volunteering in a bear rescue centre provided me with firsthand experiences that I used to enrich the characters’ lives and make them authentic,” he added.

Noble said that Saving Sorya “is a beautiful story, elegantly told, which brings together a global view of conservation and an empowering true story of an inspiring female environmentalist, told through dazzling manga art and watercolours. Jeet has crafted every illustration to immerse the reader, just as Manon draws the reader in completely with her vivid, deliberate prose.”

The prizes were judged by a panel of 12 librarians, who selected winners from a shortlist of seven for the writing medal and six for the illustration award.

In a separate prize, children from reading groups in schools and libraries also voted for their preferred winners, who each received a medal. This year’s “shadowers’ choice” award for writing went to I Must Betray You by Ruta Sepetys, who previously won the headline Carnegie medal in 2017 for Salt to the Sea. Joe Todd-Stanton won the shadowers’ illustration medal for The Comet.

The Carnegie medals were established in 1936 for writing and 1955 for illustration, open to books in the English language. This means that in theory translations into English have always been eligible. Previous winners of the writing award include Arthur Ransome, CS Lewis, Penelope Lively, Neil Gaiman, Terry Pratchett, Eleanor Farjeon and Phillip Pullman; earlier illustration winners include Raymond Briggs, Shirley Hughes and Quentin Blake.

• This article was amended on 27 June 2023 to add some names to the list of previous Carnegie writing award winners for greater context.