"Against this background, the Board's carefully considered view is that to issue a certificate to Manhunt 2, on either platform, would involve a range of unjustifiable harm risks, to both adults and minors, within the terms of the Video Recordings Act, and accordingly that its availability, even if statutorily confined to adults, would be unacceptable to the public."
The Video Recordings Act allows game distributors to appeal the decision, which we're certain Rockstar will want to take advantage of. This is a monumental decision in the games industry; the last video game to be refused a classification was Carmageddon in 1997 (which was later overturned by an appeal).
Gallery: Manhunt 2
[Via press release]