J.K. Rowling has taken everyone by surprise by unveiling a new book for children, The Ickabog, which she will be posting online on a daily basis free of charge.

The story will have its own website, where the author explained on Tuesday: “I had the idea for The Ickabog a long time ago and read it to my two younger children chapter by chapter each night while I was working on it.

“However, when the time came to publish it, I decided to put out a book for adults instead, which is how The Ickabog ended up in the attic. I became busy with other things, and even though I loved the story, over the years I came to think of it as something that was just for my own children.”

Explaining her change of mind, Rowling said: “Then this lockdown happened. It’s been very hard on children, in particular, so I brought The Ickabog down from the attic, read it for the first time in years, rewrote bits of it and then read it to my children again. They told me to put back in some bits they’d liked when they were little, and here we are!

“The Ickabog will be published for free on this website, in instalments, over the next seven weeks, a chapter (or two, or three), at a time. It isn’t Harry Potter and it doesn’t include magic. This is an entirely different story.”

She addressed children: “The most exciting part, for me, at least, is that I’d like you to illustrate The Ickabog for me.

“Every day, I’ll be making suggestions for what you might like to draw. You can enter the official competition being run by my publishers, for the chance to have your artwork included in a printed version of the book due out later this year. I’ll be giving suggestions as to what to draw as we go along, but you should let your imagination run wild.”

A few weeks ago, she suggested to her children that she might retrieve it from her loft.

“My children, 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,” the author said.

“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....”

In a tweet she said: “Opening the box was like opening a time capsule. Most of the story was handwritten, but bits had been typed up. When I put it into some kind of order (I’m not renowned for my filing skills) I had a patchwork first draft.”

Snippets of pages she posted on Twitter revealed the names of characters called Bertram and King Fred.

The story is about truth and the abuse of power, she said.

“To forestall one obvious question: the idea came to me well over a decade ago, so it isn’t intended to be read as a response to anything that’s happening in the world right now. The themes are timeless and could apply to any era or any country.”

The Ickabog 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”.

When the culture secretary Oliver Dowden tweeted in support of Dominic Cummings, Boris Johnson’s embattled aide, and said, “Dom Cummings followed the guidelines and looked after his family. End of story,” the cabinet minister was put firmly back in his place by the author, “I know ending stories and this ain’t it, chief.”