Harry Potter returns, this time in a new J.K. Rowling play

Informal casting has begun for a new Harry Potter play by J.K. Rowling said to be about the lives of Harry Potter's parents, Lily and James.

Called Harry Potter and the Cursed Child, it is due to open at the Palace Theatre in London's West End in the summer of 2016. Although not a musical, it is expected to eclipse box office records for blockbuster West End productions such as The Lion King, The Phantom of the Opera and Cats.

Rowling released the first details of the stage production, via Twitter on Friday.

It was "an untold part of Harry's story" but, she said, "I don't want to say too much more, because I don't want to spoil what I know will be a real treat for fans. However, I can say that it is not a prequel!"

"To answer one inevitable (and reasonable!) question ... I am confident that when audiences see the play they will agree that it was the only proper medium for the story."

Rowling says Harry Potter and the Cursed Child, the first-ever play about the boy wizard, was a collaboration with playwright Jack Thorne, director John Tiffany and Sonia Friedman, a West End and Broadway theatre producer. 

Tiffany, who directed, and won awards for, the musical Once and the plays Black Watch and Let The Right One In, will work with choreographer Steven Hoggett, who won an Olivier Award for Black Watch, and set designer Christine Jones.

The play is said to explore what happened to Lily and James before they were killed by Lord Voldemort, prompting Harry to live a life of misery with his in-laws, the Dursleys, at 4 Privet Drive. It may also explore the friendship between Lily and Severus Snape at Hogwarts.

On Wednesday, on the official Harry Potter web site Pottermore, Rowling revealed the origins of why Petunia and Vernon Dursely disliked their nephew so intensely.

Petunia, a muggle, was "forever embittered by the fact that her parents seemed to value her witch sister more than they valued her".

And Vernon was "apt to despise even people who wore brown shoes with black suits; what he would make of a young woman who spent most of her time wearing long robes and casting spells, Petunia could hardly bear to think."

Rowling descibed Uncle Vernon and Aunt Petunia as "reactionary, prejudiced, narrow-minded, ignorant and bigoted"; qualities that Rowling says are her "least favourite things".

The seven Harry Potter novels have sold more than 450 million copies around the world and been made into eight films.