Usually concerned with top secret matters affecting national security, Britain's eavesdropping spy agency GCHQ was also on the lookout for leaks of a yet-to-be-published Harry Potter book, its publisher has revealed.

Shortly before the publication of one of the volumes in J.K. Rowling's seven-part wizarding saga, with millions of fans worldwide at a fever pitch of anticipation, publisher Nigel Newton received an unexpected phone call.

"I remember the British spy eavesdropping station GCHQ rang me up and said 'we've detected an early copy of this book on the Internet,'" Newton told Australia's ABC Radio in an interview last week that gained attention in Britain on Sunday.

"I got him to read a page to our editor and she said 'no, that's a fake'," said Newton, founder and chief executive of Potter publishing house Bloomsbury, describing the spies as "good guys."

"We do not comment on our defense against the dark arts," a spokesman for GCHQ said.

As any Potter reader will know, Defense Against the Dark Arts is a subject taught at Hogwarts School of Witchcraft and Wizardry, in which pupils learn how to defend themselves and fight back against the evil deeds of Dark Wizards.

This year marks the 16th anniversary of the introduction of the Harry Potter series into China. The book is immensely popular in the country, so much so that a number of unauthorized Potter books have been published, media reported.