JK Rowling has asked Harry Potter fans to keep the plot of The Cursed Child secret as the new play about the boy wizard prepares for its first preview.

Ms Rowling told fans that they had been 'amazing' at keeping the secrets of her Harry Potter books safe in a video on YouTube.

She then asked them to do the same again to avoid spoiling the new play for others.

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The play begins previews in London's West End tomorrow.

She said: "You've been amazing for years at keeping Harry Potter secrets so you didn't spoil the books for readers who came after you so I'm asking you one more time to keep the secrets and let audiences enjoy The Cursed Child with all the surprises that we've built into the story."

The new play by Jack Thorne, which is based on a story by him, Ms Rowling and John Tiffany, will get its world premiere in London's West End on July 30.

A published version of the play will be available to buy on July 31.

The two-part play is set 19 years after the books ended and tells the tale of a middle-aged Harry Potter and his middle child Albus Severus.

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The plot for the play reads: “While Harry grapples with a past that refuses to stay where it belongs, his youngest son Albus must struggle with the weight of a family legacy he never wanted.

“As past and present fuse ominously, both father and son learn the uncomfortable truth: sometimes, darkness comes from unexpected places.”

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The printed version of the play will be available to buy on July 31.