Microsoft is investigating how one of the most hotly anticipated Xbox titles of the year appeared on the net three weeks ahead of its official release.
Halo Reach, due to be launched on 14 September, appeared on file-sharing sites such as The Pirate Bay.
The first-person shooter has already been downloaded hundreds of times.
Reports suggest that the leak occurred after the game was made available to select reviewers and journalists via Microsoft's online Xbox store.
"We are still investigating details surrounding a claimed leak of Halo Reach and have nothing further to share," Microsoft said.
Within hours of being available, videos of the game began to appear on YouTube.
The firm takes a dim view of games' piracy.
In 2009, it cut thousands of gamers off from its online gaming service Xbox Live for modifying their consoles to play pirated games.
Stephen Toulouse, director of policy and enforcement for Xbox Live, warned on Twitter that people attempting to play unauthorised copies of Halo Reach could face similar sanctions.
"As with all unauthorised play on Xbox Live, anyone playing any unauthorised title runs the risk of account permaban," he wrote on Twitter.
"Remember, legit store-bought copy runs no risk of ban. Key word: unauthorised. We have ways of knowing."
The Halo Reach leak is not the first time that the popular video game series has been targeted by pirates eager to get their hands on the game.
In 2004, the second instalment of the series appeared on file-sharing sites a month ahead of its release.
Halo 3 also was also leaked on to the internet, whilst a French retailer sold copies of Halo 3: ODST before its official launch. A copy of the game appeared on the net shortly afterwards.
Halo Reach is the final chapter of the series, developed by Bungie for the Xbox 360.
Players take the role of an elite solider battling for control of the planet Reach.