The story concerns rumours of a fierce child-eating monster with extraordinary powers which lives in the northern tip of an imaginary land known as Cornucopia.
Further chapters will be published every weekday until July 10, Rowling said on Twitter, adding in capitals: “THIS IS NOT A HARRY POTTER SPIN-OFF”.
Rowling got the idea for The Ickabog when she was writing Harry Potter and she intended it to be published after “Harry Potter and the Deathly Hallows”, the last of her seven-book Potter series.
But the mostly handwritten manuscript ended up in her attic and stayed there for a decade until a few weeks ago. She has rewritten parts of it in recent weeks.
“The Ickabog is a story about truth and the abuse of power,” said Rowling, 54. “The themes are timeless and could apply to any era or any country.”
“I’ve decided to publish The Ickabog for free online, so children on lockdown, or even those back at school during these strange, unsettling times, can read it or have it read to them,” Rowling said.
She will donate her royalties to projects and organisations helping the groups most impacted by COVID-19.