UPDATE:

BioWare has responded to the leak on its official forums . BioWare's Jesse Houston wrote the following:

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"As most of you know by now, this evening an accidental leak occurred of some early Mass Effect 3 beta code. Microsoft has advised us that through human error at their end, content which was meant for internal beta testing was flagged incorrectly and released to individuals who had signed up for the beta of the Xbox Live dashboard update. It has since been taken down and deactivated.We'd like to stress that the leaked code is in a rough, unfinished state and was not meant for public release. The content is not final, and is not reflective of the quality of the finished, polished game due for release on March 6, 2012 (March 9 in Europe).One feature that was present in this early build is a set of game play modes that are in development. In the beta code they are referred to as "action mode", "story mode" and "RPG mode". We want to stress that these features are still in the early stages of development and will receive considerable iteration and refinement. These modes are designed to give players an even finer degree of control over their game experience than ever before. Whether it be someone who finds the combat difficult but wants to experience the amazing story, someone who wants to focus on the action and combat game play, or fans who want the rich, story-driven RPG Mass Effect experience they've come to love – Mass Effect 3 Original story follows:A closed beta for Mass Effect 3 was mistakenly made public on Xbox Live today. The beta was available for a short time as a 1.8 GB download and featured both single-player and multiplayer modes.The download is no longer available, but some players were able to access gameplay sections and cutscenes, as well as in-game menus revealing Origin integration, game options and character selection screens.Users on the NeoGAF forums were able to capture both screenshots and video of the various modes, including character creation and Galaxy at War. The content appears to have been mostly similar to the modes we saw in September , though the beta was missing some sound effects and textures that will appear in the finished game.The download is likely the same as the demo that BioWare detailed last month . An official release date for the multiplayer beta has still not been revealed, though the single player demo is scheduled to be released in January.