The Entertainment Software Rating Board changed the rating as the game's publisher Take Two admitted it created sex scenes found in the title.

The scenes were unlocked following the installation of a patch known as Hot Coffee developed by a fan of the game.

Take Two said the scenes were also in the Xbox and PlayStation versions.

Closer look

The investigation by the ESRB followed reports about the Hot Coffee hack that let players view sex scenes in the game.

Initially Take Two said the explicit scenes were the result of modifications made to the game by Hot Coffee.

The ESRB investigation found that the scenes were in the game all along.

"What was clear to us is the fact that fully rendered content existed on the disk that was not disclosed," said Patricia Vance, ESRB president.

"The publisher took the risk that a hacker could find it and it clearly put the rating at risk."

In response Take Two said it would stop making the current version of the GTA game and offer an update that made the Hot Coffee hack useless.

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It is also working on a version of the game with the sexual content stripped out that can be sold under the mature rating.

Off the shelves

The news had an instant impact on shares in Take Two, owner of GTA maker Rockstar Games, which fell 11% in after-hours trading.

The change in rating led to shops such as Wal-Mart, Target and Best Buy removing the game from their shelves as they have a policy of not stocking titles with AO ratings.

For those stores that were happy to keep selling the game Take Two said it would provide AO labels so the game can be properly marked.

A Rockstar Games spokesman said it supported a strong ratings system that guaranteed that its games would only be sold to the people for which they were intended.

The controversy over the game caught the attention of US senator Hillary Clinton who had called for an investigation into who put the scenes into GTA: San Andreas.

Following the decision by the ESRB, Senator Clinton said Take Two had "gamed the ratings system and enabled pornographic material to get into the hands of children".

San Andreas was one of the best-selling games of 2004. The title already has an 18 age rating in the UK.

The GTA series of games have created controversy in the past. Previous versions have been criticised by parent groups and lawmakers for their depictions of violence and sex.