The latest addition to one of the world\'s best selling videogame series went on sale in the UK at midnight.More than 500 stores held special openings to let gamers buy Call of Duty: Modern Warfare 3.Its predecessor, Call of Duty: Black Ops, generated more than $1bn (£620m) of global sales over the first six weeks of its launch.However, troubles at the studio behind the new title have threatened to overshadow its release.The latest game follows on from events in Call of Duty: Modern Warfare 2. Players take on the role of special forces resisting a Russian invasion of the United States and Europe.The campaign is set across locations in England, France and Germany as well as Africa, India and the Middle East.Modern Warfare 3 is the first title in two years to be developed by Infinity Ward, the creator of the Call of Duty series.Critics are keen to discover if staffing changes at the California-based studio affected the product.Activision fired Infinity Ward\'s founders, Jason West and Vincent Zampella, in 2010 accusing them of misconduct. The firm later described them as \"insubordinate and self-serving schemers\" who had negotiated with rivals while under contract.The two men sued their ex-employer seeking unpaid royalty payments and damages. Activision countersued, alleging that the men had held secret talks with Electronic Arts.A court in Los Angeles is set to consider the case in May 2012.Mr West and Mr Zampella went on to set up a new studio, Respawn Entertainment, which signed a publishing and distribution rights deal with EA.Several of Infinity Ward\'s other employees also moved to the new studio. Respawn\'s website reveals that 40 members of its staff previously worked on the Modern Warfare series.Activision brought in one of its other teams - Sledgehammer Games - to help the remaining Infinity Ward workers complete Modern Warfare 3.Several members of Sledgehammer\'s team had helped develop the highly regarded Dead Space series, helping restore gamers confidence in the project.There is now huge interest in what they have been able to deliver.A trailer of the new game, posted on YouTube on 21 October, has already attracted more than four million hits.IHS Screen Digest predicts the title will sell more than 20 million copies worldwide before the end of the year, despite competition from EA\'s rival title, Battlefield 3, which was released over the last fortnight.\"It\'s not just about day one sales for these types of games - there\'s a heavy online-multiplayer aspect,\" said Steve Bailey, an analyst at Screen Digest.\"Gamers pay for enhanced subscription services such as Call of Duty: Elite\'s extra social/profile features, or direct-purchase other content such as map packs.\"The true test will be taking place over the next six months to a year, where we see who can support the gaming community best, retaining and engaging an audience in an ongoing fashion.\"Not everyone has been willing to wait. French media reported that thousands of copies were stolen in a Paris heist, and pirated versions of the game have been spotted online.Other titles fighting for a place at the top of the Christmas games charts include Skyrim, the fifth entry in the Elder Scrolls role-playing series, and the action adventure Batman: Arkham City.