Pottermania: JK Rowling hails fans for keeping Harry Potter and the Cursed Child secrets

Arriving at the Palace Theatre in London’s West End for the opening gala, she said she was delighted fans had kept the secrets of the new instalment in the series, adding: “They’ve been amazing, they’ve been incredible, and you know what it is?

“It is the most extraordinary fandom so I’m kind of not surprised they didn’t want to spoil it for each other but I’m so happy we got here without ruining it.”

Rowling added she was excited to see the play again as it had been a long time since she walked a red carpet at a Harry Potter premiere, and revealed she hopes the play will travel as widely as possible.

Asked if it will head to Broadway, she said: “I’d love it to go wider than that. I’d like as many Potter fans to see it as possible.”

Mayor of London Sadiq Khan spoke of his pride that the play was launching in the capital.

He said: “Many thanks to JK Rowling for ensuring the premiere is here,” and added that he was a “big fan” of Harry Potter, saying: “What’s important is that the world premiere is here in London, and we should be really proud.”

Director John Tiffany said he was excited to finally be officially opening the show after nearly eight weeks of previews.

He thanked fans for keeping the secrets so far, comparing sharing plot details with opening kids’ Christmas presents in November: “Why would you do that?”

“Pottermania” will reach fever pitch as the Cursed Child opening in London is immediately followed by the midnight release of the play’s script book, allowing fans across the globe to find out what happens next to the boy wizard and his friends.

As the play officially opens at the Palace Theatre after nearly eight weeks of previews, audience members will expect a magical experience after rave reviews from critics.

Set 19 years after the events of the seventh and final book, The Cursed Child brings back Harry Potter, now grown up and an employee at the Ministry of Magic.

Harry and his wife Ginny Weasley wave off their youngest son Albus Severus to their old wizarding school, Hogwarts - but once there, Albus struggles with the weight of his family legacy and goes to extreme and dangerous lengths to right the wrongs of the past.

Daily Telegraph critic Dominic Cavendish said that “British theatre hasn’t known anything like it for decades”.

He raved: “It’s a triumph. Not an unqualified one - there are some quibbles - but in all key respects, it grips, it stirs, it delights.”

The Independent’s Jack Shepherd awarded the play five stars, writing: “It’s quite apparent this isn’t written to be either a book or a tie-in film; it’s a spectacle for the theatre, one that is filled to the brim with fan service and magical imagery that will amaze.”

The two-part play stretches over five hours and was co-devised by Rowling, written by Jack Thorne and directed by John Tiffany.

Jamie Parker plays Harry, who is joined on stage by his school friends Ron Weasley (Paul Thornley) and Hermione Granger (Noma Dumezweni).

Albus actor Sam Clemmett has been a big hit, as has Anthony Boyle as Draco Malfoy’s son Scorpius.

Despite the script’s release and the play’s opening, audiences will still be urged to “keep the secret” as they leave.

Rowling herself has asked fans not to reveal any plot details after seeing the show.

The Cursed Child is currently sold out, but another 250,000 tickets are set to go on sale next week as the play’s run has been extended until December 2017.