In an interview with American news channel CNN, Rowling admits she wrote the story to celebrate her 50th birthday.
She also said her fear of small spaces may have inspired her to keep Harry locked up in a cupboard in her hit book series.
Speaking about her secret manuscript, she said: “The theme of my 50th birthday was come as your own private nightmare. And I went as a lost manuscript. I wrote over a dress most of that book… I don’t know whether it will ever be published, but it’s actually hanging in the wardrobe currently.”
Rowling also revealed: “My personal worst fear is powerlessness and small spaces. When you think about the boy trapped in the cage bed, he is totally voiceless, nobody was speaking for him. I suppose why did I put Harry in the cupboard?”