Stoke 's Sam Bangoura (l) and Julian Gray compete for the ball

Forssell bundled the ball home at the far post after Martin Latka had flicked on Jermaine Pennant's corner.

Stoke had plenty of chances but could not beat Birmingham keeper Maik Taylor.

Taylor tipped over Carl Hoefkens' long-range effort and also kept out Michael Duberry's header before denying Sam Bangoura from close range.

Birmingham restored midfielder Nicky Butt to their line-up in one of three changes by boss Steve Bruce.

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Butt, who walked out of the team hotel when he found out he had not been picked to face West Ham last week, returned to the side along with Emile Heskey and Damien Johnson, who had completed their bans.

It was Heskey who had the first chance of the game when the ball broke to him on the edge of the area and he fired in a low shot that Stoke keeper Steve Simonsen did well to keep out.

Stoke had made a quiet start but once Luke Chadwick got into the game on the right flank they began to threaten.

Chadwick's dangerous cross was cleared only as far as Dave Brammer, who got plenty of power behind his shot but saw it fly straight at Taylor after 18 minutes.

Bangoura, playing despite a broken hand, then shot narrowly wide from Brammer's corner as Stoke continued to press forward and only a superb save from Taylor kept out Hoefkens' fierce 30-yard shot.

Birmingham's only other first-half chance had been a free-kick that Pennant curled narrowly wide but they found themselves ahead two minutes after the interval.

Butt's powerful shot was deflected wide and from the resulting corner Latka headed the ball on at the near post for Forssell to force it over the line from close range for his 10th goal in 10 FA Cup games.

Stoke pushed forward and only Taylor stood in the way of an equaliser.

Another Brammer corner was met by a Duberry header, which was dipping under the crossbar until the Birmingham keeper intervened.

Taylor was somewhat fortunate with his next save when Bangoura directed his header straight at him from six yards after Mamadou Sidibe's cross was deflected into his path.

That was as good as it got for the Potters, however, and Johan Boskamp's side seemed to run out of ideas in the last 20 minutes.

Bangoura and Brammer both had speculative shots from the edge of the box but it was Birmingham who came closest to scoring again when Heskey's looping header just missed the target.