Microsoft has said it is trying to track down the people behind a pirated copy of the forthcoming Halo 2 game.
Halo 2 is expected to sell millions of copies
A version of the eagerly awaited Xbox game has appeared on net piracy sites and newsgroups, almost a month before making it into the shops.
In a statement, Microsoft said it was investigating the leak, adding that it was "aggressively pursuing the source of this illegal act".
Halo 2 is due to go on sale worldwide in the second week of November.
At the start of the week, the game's makers Bungie Studios said work on the title was complete and the game was ready to go into production.
The sequel is expected to be one of the best-selling games of the year.
The original title, Halo: Combat Evolved won universal acclaim when it was released in 2001, going on to sell more than four million copies.
Halo 2 is set to hit in the shops at the start of November. But its release has been overshadowed by the release of a pirated version of the game on the net.
The copy in circulation is a European version of the game on DVD, which is almost three gigabytes in size.
Microsoft, which owns Bungie, has reacted to the act of piracy by promising firm action.
"We consider downloading this code or making it available for others to download as theft," said the company in a statement.
Halo2 is one of the big games coming for the Xbox
"We are currently investigating the source of this leak with the appropriate authorities.
"Microsoft takes the integrity of its intellectual property extremely seriously, and we are aggressively pursuing the source of this illegal act."
It added that the piracy would not affect the release of Halo 2
Experts say the leak is unlikely to have an impact on sales of the game.
"Whilst the leak happened, Microsoft have acted quickly to shut it down,"
James Ashton, publisher of the Official Xbox Magazine told BBC News Online.
"Without illegally modifying your Xbox console there would be no way to make this leaked code play due to the consoles in-built security.
"Even with the modification you would not be able to take Halo 2 online, which, having played it, is a major feature of the title."
Halo is not the first high profile games to have fallen victim to piracy online.
In August, a copy of Doom 3 was leaked online ahead of its release and downloaded by thousands of people.
And last year, key parts of the code behind still unreleased PC game, Half-Life 2, made their way onto the net.