The Harry Potter author says she wrote the fairy tale for her children as a bedtime story more than a decade ago.

Set in an imaginary land, it is a stand-alone story "about truth and the abuse of power" for children from seven to nine years old and is unrelated to Rowling's other books.

Rowling said the draft of the story had stayed in her attic while she focused on writing books for adults.

She said her children, now teenagers, were "touchingly enthusiastic" when she recently suggested retrieving the story and publishing it for free.

"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," she said on Tuesday.

"'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)."

The first two chapters were posted online on Tuesday, with daily instalments to follow until July 10.

The book will be published in print later this year, and Rowling said she will pledge royalties from its sales to projects helping those particularly affected by the pandemic.

Australian Associated Press