In the world of computer games, it is the denouement to match Harry Potter and the Deathly Hallows: what will happen to Master Chief in his epic battle against the evil alien Covenant?

But as fans queued up last night for the global launch of Halo 3 — the final chapter in the blockbuster series — the game’s climax had already leaked out on the web.

Cheats uploaded the movie-like scenes on to YouTube, potentially spoiling the enjoyment of thousands of players.

The Halo games, which play on Xbox, have made hundreds of millions of dollars for its makers, Microsoft. The game went on sale across the US a day before Europe, and in New York about 500 people turned out for the midnight launch. Alex Escobar, who was one of the first in the queue, said: “It is worth it. It is time to finish this fight,” echoing the tagline for the futuristic “shooter” game.

Microsoft hosted several film-style premieres across Europe before the game went on sale, with the music star Pharrell Williams appearing at a VIP launch party at the IMAX cinema in London.

About 1,000 British stores held late-night openings, and the retail chain Game reported that people had begun queueing to buy Halo 3, priced at £40, from its Harrow store at 4pm.

Kyle Bigland, 17, a student from Manchester, was first in the queue at the Game store at the Trafford Centre, Manchester, and his five-hour wait was just the start of his night. “As soon as I get home I’m going to play Halo 3 all night until I complete it,” he said.

“I’ve already got a crate of Red Bull ready for it. Everyone in the queue is a bit giddy. They can’t wait.”

The queue outside the store contained scores of mainly teenage boys. Anthony Lally, 21, a student at Bolton University, arrived at 10pm. He said: “My friends are going to be jealous. They’re going to get it tomorrow, and I’ll have a couple of hours’ practice in and already be better than them.”

Global pre-orders have passed the 1.25 million mark, according to Microsoft, which is hoping to exceed the $125 million (£61 million) that Halo 2 took on the opening days of its sale.

Microsoft wants Day 1 sales of the new game to top £70 million, more than the opening-day box office take of any film in history.

Like the DVD of a Hollywood film, the game comes in a series of “director’s cut” versions. A £50 limited edition contains a bonus disc with a behind-the-scenes look at the game. The £70 Legendary Edition includes a Spartan helmet case.

Neil Thompson, head of Xbox UK, played down the potential damage that the YouTube spoilers could cause. He said: “Once the game is launched, there was always a danger that people will spoil it for others, but we cannot invest a whole cycle in preventing that. It’s like the Harry Potter books. Someone in the first queue will skip to the end and put in on the internet. We can’t do anything about that.”

John Haulihan, of the gaming website computerandvideogames.com, said that Halo was the “figurehead” game for the Xbox in its battle against rival Nintendo and Sony consoles. “The expectation among fans is absolutely huge,” he said.

Microsoft has pledged to turn round $7 billion of losses from the Xbox division since the 2001 launch of the console. It hopes to sell six million copies of Halo 3 over six months and boost total Xbox sales to 16 million.

Nintendo’s Wii, which has a “magic wand” that lets players mimic human movement, has stolen a march on its rivals in the console wars. It topped the August sales chart, selling 403,600 units, compared with Sony’s sale of 130,600 PlayStation 3 consoles and 276,700 for the Xbox 360.

Although Master Chief may have fought his last alien horde, the Halo franchise is too lucrative for Microsoft — which hopes to make $1 billion from the series by the end of the year — to kill. The company has two more Halo-related games in production: one, Halo Wars, is an “action-strategy” title and the other is believed to be a new type of interactive game.