Best-selling game Grand Theft Auto: San Andreas is being investigated in the US over reports that it contains sexually explicit mini-games hidden in its code.
The investigation centres on the PC version of GTA: San Andreas
The controversy surrounds a download available on the net which is said to unlock secret sex scenes.
Game makers Rockstar said they were complying with the inquiry, by the industry body that sets age ratings.
If the findings were to lead to an adult-only age rating, it could limit sales from major retail outlets.
The Grand Theft Auto series of crime games have proved phenomenally popular. They are credited with kick-starting the driving-and-shooting genre of gaming.
The PlayStation 2 version of San Andreas sold more than a million copies in the first nine days that it was on sale in UK last year.
But the game has also attracted criticism from those who say it encourages gratuitous violence as players immerse themselves in an underground world of LA gangs and gun-ridden ghettos.
Now it is under fire over whether the game contains hidden graphic sexual content.
Software code developed by GTA Dutch fan Patrick Wildenborg is said to have unlocked mini-games in the PC version of San Andreas that allows players to make game characters perform sexually explicit acts.
The industry body which regulates games in the US, the Entertainment Software Rating Board (ESRB), said it had opened an investigation into the so-called Hot Coffee modification.
The aim is to "determine if there has been a violation of ESRB Rules and Regulations requiring full disclosure of pertinent content," ratings group President Patricia Vance said in a statement.
The results of the investigation could determine if San Andreas retains its M rating, which is given to games for ages 17 and older, or is reclassified as an adults-only title.
This kind of rating could lead big stores in the US to limit sales or take it off the shelves altogether.
Rockstar has said it is complying with the investigation.
"We also feel confident that the investigation will uphold the original rating of the game, as the work of the mod community is beyond the scope of either publishers or the ESRB," said Rockstar in the statement.
The fan at the centre of the controversy, Patrick Wildenborg, has said he did not create the sex scenes, but enabled them with his modification.
"But all this material is completely inaccessible in an unmodded version of the game," he said in a statement on his website.
"It can therefore not be considered a cheat, Easter Egg or hidden feature But is most probably just leftover material from a gameplay idea that didn't make the final release."
The PlayStation 2 version of San Andreas was one of the best-selling games of 2004. The title already has an 18 age rating in the UK