Thousands of filesharers are understood to have got their hands on the game.
Earlier this month, the controversial game was rejected for a second time by the British Board of Film Classification (BBFC), despite revisions made after a ban in June.
Developer Rockstar had launched an appeal to the BBFC Video Appeals Committee against that decision, which was suspended while the revised version was considered for classification.
Plans to release the game in the US were affected after the Entertainment Software Rating Board (ESRB) issued the game with an 'Adults Only' rating, which would have barred the game from sale in most major US retailers.
A modified version of the game was given an updated 'Mature' rating by the ESRB in August, but the BBFC is not convinced that the changes are sufficient.