JK Rowling has revealed the back story behind the Dursleys’ violent dislike of their nephew Harry Potter in a new piece of writing for her website, Pottermore – as well as the reasoning behind her decision to let Petunia Dursley walk away from Harry without a word of kindness in the final novel.

Some readers, Rowling writes, “wanted more from Aunt Petunia during this farewell”. At the start of Harry Potter and the Deathly Hallows, Harry leaves his family behind for good - his cousin Dudley shakes his hand, his uncle Vernon roars “I thought we were on a tight schedule”, and his aunt gives him a final look. Rowling writes in the novel that “for a moment Harry had the strangest feeling that she wanted to say something to him: she gave him an odd, tremulous look and seemed to teeter on the edge of speech, but then, with a little jerk of her head, she bustled out of the room after her husband and son.”

In her new piece for Pottermore, Rowling describes the Dursleys as “reactionary, prejudiced, narrow-minded, ignorant and bigoted; most of my least favourite things”, and says that she wanted to suggest that “something decent (a long-forgotten but dimly burning love of her sister; the realisation that she might never see Lily’s eyes again) almost struggled out of Aunt Petunia” during the final farewell, “but that she is not able to admit it, or show those long buried feelings”.

Despite disappointment from some readers, Rowling writes on Pottermore, she believes that Petunia behaves in the novel “in a way that is most consistent with her thoughts and feelings through the previous seven books”.

Petunia Dursley, the sister of Harry’s mother, and her husband Vernon take their nephew in when his parents are murdered, raising him alongside their own son Dudley, but forcing him to live in the cupboard under the stairs. Writing on Pottermore, Rowling explains how their cruel treatment dates back to Petunia’s jealousy of her sister Lily’s magical abilities when they were growing up, and Vernon’s dislike of anything out of the ordinary.

“Vernon was apt to despise even people who wore brown shoes with black suits,” writes Rowling, so he was unimpressed to discover the existence of Petunia’s witch sister Lily.

When Petunia and Vernon first met Lily’s boyfriend James, the couples fell out, Rowling reveals. Vernon attempted to patronise James by suggesting that wizards live on unemployment benefit, and grew angry when James told him of the solid gold his parents had in the wizarding bank Gringotts. They never made up, Vernon describing James as “some kind of amateur magician” and choosing not to attend the Potters’ wedding.

When a letter arrived from Dumbledore with news of the Potters’ murder, Petunia “felt she had no choice” but to take Harry in, but did so grudgingly, writes Rowling, “and spent the rest of Harry’s childhood punishing him for her own choice”. Vernon’s dislike of Harry, she adds, “stems in part, like Severus Snape’s, from Harry’s close resemblance to the father they both so disliked”.

Rowling also reveals where the characters’ names came from – Vernon being “a name I never much cared for”, and Petunia “the name that I always gave unpleasant female characters in games of make believe I played with my sister, Di, when we were very young”.

The surname Dursley, meanwhile, is drawn from the Gloucestershire town of the same name, not far from where Rowling was born.