JK Rowling is to make a surprise return to the fantastical world of witches and wizards for her screenwriting debut – but there will be no sign of Harry Potter.

Fans of the bestselling series about the teenage wizard will nevertheless be thrilled that his fictional world is to be expanded with a series of new films, starting with Fantastic Beasts and Where to Find Them.

Revealing the news on her Facebook page, the British author said she was going to return to the world of wizardry only “if I had an idea that I was really excited about; and this is it”.

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Warner Bros Entertainment approached Rowling with the idea of turning Fantastic Beasts, the textbook Harry and co study in Hogwarts, into a film.

She said it was a “fun idea” but would find it difficult for another author to take on the book’s protagonist, Newt Scamander. “As I considered Warner’s proposal, an idea took shape that I couldn’t dislodge. That is how I ended up pitching my own idea for a film,” Rowling said. “Having lived for so long in my fictional universe, I feel very protective of it and I already knew a lot about Newt.”

The story will start in New York, 70 years before the start of Harry Potter’s adventures in The Philosopher’s Stone, but the “laws and customs of the hidden magical society will be familiar to anyone who has read the Harry Potter books or seen the films”.

Rowling said: “Although it will be set in the worldwide community of witches and wizards, where I was so happy for 17 years, Fantastic Beasts and Where to Find Them is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world.”

Kevin Tsujihara, the chief executive of Warner Bros Entertainment, said the company was “honoured” that Rowling had agreed to the partnership. “She is an extraordinary writer, who ignited a reading revolution around the world, which then became an unprecedented film phenomenon.”

In a bid to exploit fully the potential of the new work, the company has plans to launch a Fantastic Beasts video game and a host of consumer products.

The studio will be delighted to have extended one of its most lucrative franchises, which has an inbuilt audience that will be more supportive with Rowling’s name attached. The series, which comprised eight films, grossed more than $7.7bn (£4.9bn) around the world, according to Box Office Mojo.

Last year Rowling dropped out of the Forbes World’s Billionaires list, which the magazine put down to her charitable giving and the British tax rates.

Since then, however, Pottermore, the online Harry Potter resource, has launched, she has published the bestselling The Casual Vacancy, and has now signed this deal with Warner Bros, which might bring her back on to Forbes’s list before too long. The book is to be turned into a television miniseries by the BBC, which will then be distributed around the world by Warner Bros.

Earlier this year, Rowling was also revealed to be the author of The Cuckoo’s Calling, a detective book published under the pseudonym of Robert Galbraith. After she was unmasked the well-reviewed but modest-selling book became an instant bestseller.

Wizard creatures: From the book of fantastic beasts

Basilisk

This giant serpent can kill with a single glance; the Ministry of Magic grants it an XXXXX classification for being a known wizard-killer. But Harry Potter fights and despatches the basilisk in Hogwarts’ Chamber of Secrets.

Ukrainian Ironbelly

The largest breed of dragon in the Potter world. It guards the vault belonging to evil Bellatrix Lestrange in Gringotts bank.

Hippogriff

One of the series’ best-loved creatures, Buckbeak the hippogriff is an eagle-horse hybrid. His finest moment is his attack on Draco Malfoy after the slimy student shows him disrespect.

Phoenix

Hogwarts headmaster Albus Dumbledore’s phoenix, Fawkes, saves Harry’s life and helps his master fight Lord Voldemort.

Merpeople

Live underwater and speak Mermish. Hogwarts’ own colony torments Harry during a Triwizard Tournament task in The Goblet of Fire.