The Ickabog is her first children's story not to be linked to Harry Potter. She wrote it over a decade ago for her own children and has now dusted it off.
It's for "children on lockdown, or even those back at school during these strange, unsettling times", she said.
She had previously referred to it only as an unnamed "political fairytale".
Chapters of The Ickabog are being published daily until 10 July on The Ickabog website.
The first two chapters, which went online on Tuesday, introduced King Fred the Fearless, ruler of Cornucopia, and five-year-old Bert Beamish.
Readers also learned about the myth of a fearsome monster called The Ickabog, which is "said to eat children and sheep".
The author said she originally intended to release the story after the seventh and final Harry Potter novel came out in 2007.
But she decided to take a break from publishing, and put the manuscript in her attic.
"Over time I came to think of it as a story that belonged to my two younger children, because I'd read it to them in the evenings when they were little, which has always been a happy family memory," she wrote on her website.
A few weeks ago, she suggested to her children that she might retrieve it from her loft.
"My now teenagers were touchingly enthusiastic, so downstairs came the very dusty box, and for the last few weeks I've been immersed in a fictional world I thought I'd never enter again.
"As I worked to finish the book, I started reading chapters nightly to the family again.
"This was one of the most extraordinary experiences of my writing life, as The Ickabog's first two readers told me what they remember from when they were tiny, and demanded the reinstatement of bits they'd particularly liked (I obeyed)."
It was written to be read aloud, but is suitable to be read alone by children between seven and nine, she said.
It will be published as an actual book in English in November, with all author royalties going "to help groups who've been particularly impacted by the pandemic".