Halo: Reach fans have been urged to "go dark" following the leak of the Xbox 360 exclusive onto the internet weeks before its official release.
Moderators of the US studio's forum issued the warning after a hacker group claimed to have obtained the game from Microsoft's own website.
"Posting to Discuss, Request, or Link To information or campaign spoilers from illegitimately obtained sources - including illegitimately obtained copies of Halo: Reach - before the game is officially released will be considered a leak and will be treated as such," moderators said on Bungie's forum.
"The moderation team is doing their best, but if you want to stay 100 per cent spoiler free, our best advice is to 'go dark' by staying off all internet gaming forums until you have played through the game!"
Halo: Reach, due out 14th September, was made available to download by reviewers from the official Xbox Live Marketplace website.
The 7GB download is priced at 99999 MS Points (Ł856.66).
The hackers apparently bypassed Microsoft's security and acquired the science fiction shooter.
Microsoft will no doubt be seething that the Xbox 360's biggest Christmas exclusive is playable ahead of its release despite the hackers' promise that they're "not really planning" to release the title publicly on the internet.
Somebody obviously planned to: the game is currently available on file sharing sites.
"We are aware of claims being made regarding a security exploit related to Halo: Reach and are aggressively investigating the matter," a spokesperson told IGN.
"We have no further details to share at this time."