Harry Potter author JK Rowling has returned to the world of her teenage wizard to explain the background of one of her most evil characters.

She has written more than 5500 words for her Pottermore website going into the history of Azkaban prison and the story of Dolores Umbridge – a witch played in the films by Imelda Staunton.

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Rowling explains that Umbridge, who tormented the teenage wizard as a teacher at Hogwarts, is based on a real person whom she “disliked intensely on sight”.

She said: “The woman in question returned my antipathy with interest. Why we took against each other so instantly, heartily and (on my side, at least) irrationally, I honestly cannot say.”

Rowling said Umbridge was “one of the characters for whom I feel the purest dislike”, adding: “Her desire to control, to punish, and to inflict pain, all in the name of law and order, are, I think, every bit as reprehensible as Lord Voldemort’s unvarnished espousal of evil.”

She has previously used the website to update the Potter stories and offer fans a glimpse into the lives of her characters after the books ended.

In one, she wrote about a school reunion that saw a now 34-year-old Harry back at Hogwarts with “threads of silver” in his black hair and a mysterious cut over his cheekbone which signifies his membership of a top-secret group of wizards called the Aurors.

The author, who lives in Edinburgh, recently published her second crime thriller under the pseudonym Robert Galbraith.

The real identity of Robert Galbraith was revealed last year and sent the first novel, The Cuckoo’s Calling, to the top of the book charts.

She accepted a substantial donation to charity from the law firm which breached her confidentiality by revealing she was writing under a pseudonym.

Only a handful of trusted advisers, family and friends were initially aware of Galbraith’s true identity until the story made headlines worldwide.