The stands are empty; the pitch, still sodden and scarred from recent international rugby union clashes, is unused. Twickenham stadium in south London has hosted many major tournament finals – but none like this. Today's matches take place in a darkened hall within the stadium's towering walls, on the battlefields of multimillion-selling first-person shoot-'em-up Call of Duty. Over two days, 26 teams from across Europe are competing for a place in the World Championship, held in LA at the end of the month. All the matches use the same setup: two teams of four players competing for the most "kills" in one of the game's fast-paced military-themed gunfights. A slouching gaggle of twentysomethings have come to watch these qualifying battles live, though most of them don't arrive until after lunchtime. If glitzy video game tournaments are the future of sport, the future hasn't quite reached south London yet.

But the emerging world of professional video game competition – or eSports – is serious business. This year's Call of Duty tournament has a prize purse of $1m and the winning team takes home $400,000. Elsewhere, last year's prize pot for the fantasy-themed League of Legends championship – arguably the most popular online game in the world today – was $2m, attracting sponsorship from heavyweight brands such as Coca-Cola and American Express.

League of Legends fans watch a live game. Photograph: AFP/Getty Images

Last year's League of Legends final was hosted in Los Angeles' Galen Center – a location more accustomed to nationally televised basketball matches – watched live by 10,000 fans and online by more than 32 million people throughout the day, peaking at eight million simultaneous viewers. In South Korea, the sci-fi strategy game Starcraft has been referred to as the country's national sport – the top players are adored by millions of fans, and cable channels cover the game's professional tournaments. Call of Duty's European qualifiers may not have made it to the pitch yet, but they are at least inside the stadium, a symbolic stride towards a certain kind of sporting legitimacy.

Call of Duty is a cultural juggernaut, produced in annual iterations that, as publisher Activision is only too eager to proclaim, routinely generate more profits than any other entertainment release in a year. Each Call of Duty title is a game of two halves: a six-hour cinematic storyline for a single player, and a vast and complicated online competitive sports game. The Call of Duty World Championship is now in its third year, and has grown out of the consumer game, where the best players gain recognition and are picked up by professional eSports teams, some of whom have dedicated scouts who search out the best players online.

"The time and effort these guys put in for getting to the top are crazy," says Ben Perkins, a competitor from TYT, one of the youngest teams to make it to the European leg of the championship. "They don't miss shots, of course, but it's their knowledge of the maps and communication that gives them a different sort of advantage. For example, they'll see a rival player run into a building and know that, six seconds later, they'll be in a particular spot. They'll then throw a grenade through a tiny hole in the roof to cut the other player off. It's like an entirely different game to what people play at home."

Literary profiles of young professional athletes often strain to find evidence of a rounded life, of interests outside of the training regime and the psychological obsession of preparation for the game. They strain because, in the majority of cases, there is no rounded life; the reality of top-level sports is that they demand participants' full and dogged attention, and early and total commitment to one tightly focused area of excellence. But here in Twickenham, for the moment at least, there is little evidence of this kind of absolute immersion. On the first night of the tournament, many players stay out drinking until the early hours.

Call of Duty: Ghosts.

It's not that they are being unprofessional; it's simply that the sport lacks a natural maturity. The prize pots are too few and too weighted toward the winning team for anybody to make a professional living from the game. "We tell all of the players that we work with that education comes first," says Jim Maguire, 46, manager of TCM, the team that will go on to win the European leg of the championship, and are UK favourites for the finals. "I tell them: you've got to have something to fall back on. eSports is great but it's a young person's game. You might get five years out of it. Very few go on past then so they need something to fall back on. You won't be able to retire on Call of Duty winnings."

Call of Duty championships … the prizes await.

A professional trainer would likely never say such a thing to a top-flight athlete; they would encourage the player to let go of all immediate hopes and ambitions away from the sport. There are already signs however that eSports might one day require the same level of commitment. Taiwan's top League of Legends squad, the Taipei Assassins, live and train together in the shadow of the Taipei 101 skyscraper in central Taiwan. They adhere to a strict diet and spend time every day on physical exercises to aid reflexes and general health. But, in terms of Call of Duty at least, the financial rewards aren't there to sustain quite the same level of commitment yet. "Coming second or third in these tournaments doesn't hold any major financial incentive," says Maguire.

Part of the issue may be that Call of Duty is less tailored to spectatorship than other competitive video games. Unlike League of Legends or Starcraft, Call of Duty is played from a first person perspective. "This certainly provides an intimate view on the action," explains Joe Cecot, one of the game's designers. "But it's hard to see exactly what's going on with the broader match." To aid the flow of information to spectators, and to give the game's professional commentators a tool for monitoring the flow of a game, the developer introduced a spectator mode, which outlines all enemies and friendlies in a match, even if they're obscured by a wall or building. "This allows the audience to anticipate an engagement," says Cecot. "They know something is about to go down, and the commentators can pick up on these moments. It's all about the flow of information."

Call Of Duty: Black Ops.

As the game's developers, Infinity Ward, wrestle with the task of designing tools to turn a niche spectator sport into something with broader appeal, the game's emergent semi-professional players are also forced to ask deeper questions about what these competitions mean in the fabric of their lives. Mark Bryceland, 19, is one quarter of TCM and the only Scottish player competing in this year's world championship. "I have no idea what my ambition beyond this is," he says. "It's just sort of for fun at the moment. It's a hobby that's moving to the next level; something I can make a little money from. At the core of it I suppose I just want to be known as the best at something."