The makers of the best-selling Grand Theft Auto games are "aggressively pursuing" the pirates who leaked a copy of the latest version of the game.
Game set in the fictional city of San Andreas
Copies of Rockstar's GTA: San Andreas for the PlayStation 2 have appeared on file-sharing systems on the internet.
It is the latest big title to fall victim to piracy, coming a week after the Xbox game, Halo 2, was leaked.
The Grand Theft Auto games are largely responsible for kick-starting the driving-and-shooting genre of gaming.
More than 30 million units of the Grand Theft Auto (GTA) franchise have been shipped to date, including 11.5 million copies of GTA: Vice City and 10.5 million units of GTA 3.
The latest in the franchise, San Andreas, is due to go on sale at the end of the month.
But its release has been overshadowed by internet piracy, with a copy of the American version of the game appearing online.
In a statement, Edinburgh-based Rockstar Games said a pirated version of the
game had been posted to various internet sites.
"The proper authorities are investigating the theft and are continuing to investigate all possible leads to ensure there is no further dissemination of our creative content," said the company.
"Downloading, possession and distribution of Grand Theft Auto: San Andreas, including making the game available on the internet, is theft.
"We take the theft of our intellectual property very seriously and we are and will continue to diligently and aggressively pursue this matter."
However Rockstar said the release of the game would go ahead as planned, with San Andreas hitting the shops in the US on 26 October and three days later in Europe.
The leak seems unlikely to make a big dent in sales, as only people with a modified PlayStation2 will be able to play the pirated copy.
"The phenomenon where games like GTA: San Andreas appear online before their official release is sadly becoming more common and is a very serious problem," said Stephen Pierce, editor-in-chief of Official PlayStation 2 magazine.
"We should also keep in mind that San Andreas contains mature adult content and will be sold in the UK with a BBFC 18-rating. This means the game has gone through recognised legal channels before being sold responsibly to adults."
San Andreas is not the first high profile games to have fallen victim to internet piracy.
A week ago, a copy of the eagerly awaited Halo 2 for the Xbox appeared on the internet.
In August, a copy of Doom 3 was leaked online ahead of its release and downloaded by thousands of people.
Despite this, the title still went on to sell in large numbers, debuting at the top of the UK games charts.