Rain was never going to discourage the Harry Potter faithful, who travelled from all over the world for last night’s premiere in Leicester Square.

They have been waiting for Harry Potter and the Half-Blood Prince ever since the fifth instalment in the film franchise, Harry Potter and the Order of the Phoenix, grossed $938 million (£578 million) worldwide in 2007.

In the meantime, the child actors who were plucked from obscurity in 2000 have grown up to become global stars and millionaires. Daniel Radcliffe has wowed Broadway with his full-throttle, full-frontal performance as a horse mutilator in Equus, Emma Watson has become the face of Burberry and Rupert Grint has panicked his fans with a bout of swine flu.

The new film, due for general release in Europe next Wednesday and in the US and Japan two days later, is expected to be one of the biggest grossing of the year, adding to the $4.5 billion already made by the franchise.

Understood to have cost at least $250 million, it was originally due to be released last year but was delayed because Warner Bros wanted to spread out its own hits after the success of The Dark Knight.

The delay was proof that J. K. Rowling’s boy wizard has become a crucial weapon in a multibillion-dollar tactical game between the big Hollywood studios. Michael Gubbins, a film consultant and former editor of Screen International magazine, said that studios were increasingly “putting all their eggs in one basket” by making fewer, but bigger, blockbusters.

“Timing is now a paramount concern. If you get it wrong, the business model collapses,” he said.

At a time when the DVD revenues that have underpinned spiralling blockbuster budgets are shrinking, a summer cinema release followed by a Christmas rush to buy the DVD offers the best chance of maximising profits.

None of this mattered to the bedraggled fans in Leicester Square. Julia Ferrari, 19, had travelled from Brazil to be with her boyfriend Dominiq Meyer, 21, who lives in Germany. The pair, who met through a Harry Potter fanclub in Brazil in 2005, queued from 10am hoping to meet the film’s stars.

Mr Meyer, a student, said: “Harry Potter for us is not just a movie, we met thanks to Harry Potter and we have a lot of good memories.”

Maud Laurent, 18, another student, had flown from Miami hoping to get Radcliffe’s autograph, while Hannah Cattano, 16, from Southend, slept in a tent on Monday night in Leicester Square to be at the front of the queue.

“Dad was trying to convince me not to come but I was so annoyed when they postponed the premiere and really wanted to be here,” she said.

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