JK Rowling's first book for adults will be a "blackly comic" tale about an idyllic town ripped apart by a parish council election.

Details of the novel, The Casual Vacancy – to be published later this year – were released on Thursday by her publisher.

The 480-page book will be set in Pagford, a dreamy spot with a cobbled market square and ancient abbey which becomes a town at war with itself.

Rowling, 46, has amassed a fortune estimated to be more than £620m from her seven Harry Potter books for children, plus all the spin-off rights.

This will be her first novel since her final Harry Potter book was published in 2007.

The new book, to be published worldwide on 27 September including ebook and audio formats, begins with the unexpected death of Barry Fairweather, whose demise in his early 40s leaves a space on the parish council.

Publisher Little Brown said it would be a "blackly comic, thought-provoking and constantly surprising" novel.

Jon Howells, spokesman for the book retailer Waterstones, said: "It will obviously be a major bestseller, probably the bestselling fiction title this year.

"The real surprise is the plot detail. She never revealed this much in advance of the Harry Potter novels. We were all expecting some sort of crime or mystery novel, but she has, refreshingly, surprised us all.

"The plot sounds like it will have hints of Mark Haddon and [Alexander] McCall Smith, and the promise of black comedy is very beguiling."

Rowling's Potter books have sold more than 450m copies and have been translated into 74 languages.

She published a short Potter spin-off collection of stories, The Tales Of Beedle The Bard, in December 2008.