Epic Games is investigating the leak of an unfinished version of Gears of War 3 onto the internet.
Over the weekend videos of a build of the Xbox 360 exclusive third-person shooter appeared online.
The footage contains campaign spoilers and other unreleased gameplay.
Epic and Microsoft warned fans that the footage lifted from the leaked game does not do the final game justice.
"We have seen the reports of Gears of War 3 content being propped on the Web and working closely with our security teams and law enforcement to address the situation.
"This content is not from a final build and is not representative of what fans will enjoy when the game launches worldwide on 20th September 2011."
The Gears of War 3 leak is the latest is a string of high-profile games to hit torrent sites this year.
In February near a complete build of the PC version of Crysis 2, complete with multiplayer, hit torrent sites. Reports then came in that the final version of PlayStation 3 exclusive shooter Killzone 3 was online ahead of release.