Fans who play the Xbox 360 version online before next week's release are not "authorised" to do so, said Xbox Live's director of policy and enforcement Stephen Toulouse.
"Clarification: dblchk'd with Activision. Mw3 pre-release play not authorized. So pls be patient. Playing early may impact your account! (sic)," he tweeted.
The announcement comes hours after Toulouse originally said that if copies were "legit" and obtained legally, gamers could go ahead and play.
Last week, pirated copies of Modern Warfare 3's PC version appeared online following a leak at a distribution warehouse in the US.
Call of Duty: Modern Warfare 3 is released for PC, Xbox 360 and PlayStation 3 on November 8 worldwide.
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Watch the 'Tango Down' trailer for Modern Warfare 3 below: