Blues, who have scored just four goals in the last nine games, now stand ninth in the Championship, six points off the play-offs, although with a game in hand.
BBC WM’s Blues reporter Richard Wilford
“The acquisition of Lafferty is interesting on two fronts. First, it suggests a further loosening of the financial ties that have bound Birmingham City in recent seasons – an experienced international striker does not come cheap.
“It also indicates an end on the over-reliance on striker Clayton Donaldson. He has been an indisputable starter under Gary Rowett over the last two seasons, one of the first names in the team sheet.
“But Lafferty desperately needs football ahead of Euro 2016 with Northern Ireland, and will surely have to be given opportunities at St Andrew’s.
“Will this mean a rest for Donaldson, or will Rowett go against the grain and field two strikers? “Either way it is an ambitious move at a time when the club are in danger of slipping out of play-off contention.”