Leaders Chelsea had to be content with a Boxing Day point at Birmingham after fortuitously seeing Christian Benitez's first-half effort ruled out for offside in their goalless draw.
While Chelsea enjoyed the better chances at St Andrew's it was Birmingham who had the ball in the net, Benitez poking it over the line and apparently being played onside by an injured Didier Drogba only to see the flag raised against him.
The visitors, defending a four-point lead over second-placed Manchester United, had excellent chances to win it themselves as Branislav Ivanovic, Daniel Sturridge, Alex and Florent Malouda all went close. Malouda, meanwhile, was dismissed in the closing stages after receiving a second caution.
Ivanovic went close to opening the scoring in the 11th minute when he rose to meet Malouda's corner while Juliano Belletti headed just wide.
Didier Drogba met Ivanovic's cross from deep with a powerful volley that whistled wide, and soon afterwards Sturridge blasted a shot that Joe Hart did well to save.
Birmingham had the ball in the net in the 31st minute only for it to be ruled out somewhat harshly.
With Drogba lying injured, Liam Ridgewell directed the ball towards goal - only for Benitez to stab it over the line but see the goal ruled out by the assistant referee's flag for offside. However, replays suggested the home fans' frustration was justified as Drogba played the striker marginally onside.
Alex's powerful free-kick from distance smashed into the crossbar and in added time, Sturridge volleyed wide.
Livewire Benitez was Birmingham's dangerman and he forced Malouda to foul him and concede a yellow card on the hour. From the resulting free-kick, Sebastian Larsson curled an effort towards the corner that Petr Cech did well to save, injuring himself against the post in the process.
Hart produced another excellent save from Drogba while Malouda's afternoon soured when he received a second yellow card for an ill-judged challenge on Stephen Carr.