A PC game has had its age rating raised by the Electronic Software Ratings Board (ESRB) after a player created an 'adult' modification.

Elder Scrolls IV: Oblivion was upgraded from 'Teen' to 'Mature' in the US because of a user add-on that creates topless female characters. 

Game developer Bethesda Softworks defended its product, however, insisting that it had complied fully with the ratings process and did not believe it had done anything wrong. 

"Modders have used a third-party tool to hack into and modify an art archive file to make it possible to create a mesh for a partially nude female that they add into the game," said the company in a statement.

"Bethesda did not create a game with nudity and does not intend that nudity appear in Oblivion."

The developer explained that the ESRB had insisted on the 'Mature' rating because partial nudity can be created by modders in the PC version, and the game exceeds the violence of the original 'Teen' rating.

However, the company claimed that it told the ESRB how much violence was in the game during the ratings process.

"Bethesda made what it believes was a full, accurate and comprehensive submission on Oblivion to the ESRB months before the game's release," the company said.

"Nothing was hidden from the ratings agency and no effort was made by Bethesda to lobby or influence the agency for any particular rating."

The controversy follows a similar outcry over the Hot Coffee mod for Grand Theft Auto: San Andreas, which was also published by Oblivion's publisher 2K Games.

The modifications do not affect the Xbox 360 version of the game.