JK Rowling has joked she felt like a "relapsing addict" after writing a new Harry Potter prequel for a charity auction.

Although the author’s new story only runs to 800 words it is expected to raise up to £1 million for the English PEN and Dyslexia Action charities when it goes under the hammer on 10 June.

However, Rowling - who has vowed there will be no new full Harry Potter novel - says the experience of returning to the boy wizard left her with a peculiar feeling.

“Although I did feel a bit like a relapsing addict as I sat down to write, the words poured from my pen with frightening ease," she admits on her website.