British censors are seeking a judicial review to block the sale of controversial video game Manhunt 2.
Last week developer Rockstar won a hearing at the Video Appeals Committee to have a ban on the title lifted.
But the British Board of Film Classification said that decision was based on an incorrect interpretation of the Video Recordings Act.
The game was first banned in June, for its "sustained, casual sadism". A revised version was also banned.
"The VAC judgement, if allowed to stand, would have fundamental implications with regard to all the Board's decisions, including those turning upon questions of unacceptable levels of violence," the BBFC said in a statement.
Rockstar said it was disappointed that the BBFC had decided to appeal the VAC judgment.
Strauss Zelnick, chairman of Rockstar's publisher Take Two, said: "We believe the VAC decision was correct and do not understand the BBFC's decision to expend further public resources to censor a game that contains content well within the bounds established by the BBFC's 18-plus ratings certification."
When the game was first refused certification, effectively banning it, David Cooke, director of the BBFC, said the game featured "unremitting bleakness and callousness of tone".
He added: "There is sustained and cumulative casual sadism in the way in which these killings are committed, and encouraged, in the game."
The title has become one of the most controversial games in recent times.
At the VAC appeal hearing, Fred Hasson, president of developer association Tiga, said he was "surprised at how tame [the title was] compared to some very graphical scenes I've seen in other games which have received certification".
He added: "I expected it to be a lot worse... I can't believe that this has been singled out as something that is worth banning."