Harry Potter vanquishes the big delay

As the stars of the latest Harry Potter film posed for the cameras in London's Leicester Square tonight, leaders of the British entertainment industry said it could soon become the most successful film yet in a franchise which is already the most valuable in box office history.

The first five instalments have grossed almost $4.5bn (£2.7bn) in box office sales, outstripping both James Bond and Star Wars, and after being delayed for eight months Harry Potter and the Half-Blood Prince looks set to boost the Warner Bros coffers still further.

Justin Johnson of the British Film Institute said the film had already generated an "unprecedented" level of interest, with more than £215,000 worth of advance tickets already sold for the first month of screenings at the IMAX in London. It seems certain to overtake the previous film in the series, which took just over £400,000 at the IMAX.

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He predicted that the final few films in the series were likely to be the most successful of all, due to the "snowball effect" of the franchise and its ability to suck in more and more followers the longer it continues.

"You can't help but think that this film could be an absolute smash," he said. "There's a whole generation of people now who've been brought up reading the books and watching the films, and as it goes on more people are tempted to start at the beginning and follow the series all the way through. It's just impossible to ignore Harry Potter really."

Rupert Gavin, the CEO of Odeon cinemas, said the excitement and anticipation surrounding the film was "very high" and that he expected it to build on the success of the previous instalments.

Producers Warner Bros have certainly pulled out all the stops this time around. The film's budget was $250m (£154m), double the amount spent making the first film, and the company has reportedly channelled a further $155m (£96m) into marketing and distribution.

The hype surrounding any Harry Potter film is huge, but fans' expectations for The Half-Blood Prince were raised even further by Warner Bros' decision to push its release date back by eight months. It was originally due to reach cinemas in November last year, but the company postponed it until this summer in the hope that children would be tempted to see it multiple times during the holidays.

In a statement released last August, Warner Bros' president and chief operating officer Alan Horn admitted that the decision had been taken to allow the company to make as much money as possible from the film, describing the summer season "an ideal window for a family tent pole release".

The company also felt that separating the film's release date from The Dark Knight, another of its huge blockbusters, made good business sense. It is hoping to repeat the success of the previous film, Harry Potter and the Order of the Phoenix, which was released in July 2007 and made more than £578m at the box office – almost as much as the first film, which grossed more than £600m and is still the franchise's most successful.

The decision did not prove popular with the film's director David Yates, who admitted in a recent interview that the delay was met with a "huge sense of disappointment" on set.

"It was not something I warmed to initially," he said. "At the time, I was so adrenalised, I was so caught up in the process of getting the film in on deadline and making the movie on a certain schedule, and then the decision to delay was a huge anticlimax. There was a huge sense of disappointment, I must say."

The announcement, which came just two weeks after the film's trailer was released, also sparked anger among the series' huge army of young fans, who began to send executives at Warner Bros hate mail voicing their disgust. One reportedly wrote: "I hope you choke on your own saliva."