Arsenal kept the pressure on the two Manchester clubs at the top of the Barclays Premier League as they overwhelmed Birmingham 3-0 at rainswept St Andrew's.
A Robin van Persie free-kick and Samir Nasri's 13th goal of the season put the visitors in control before an own goal from Roger Johnson added to City's misery.
The Gunners remain in third spot, two points adrift of United and City, after a sixth away success of the season.
Arsenal were worthy winners and carved out a host of gilt-edged opportunities. Skipper Cesc Fabregas pulled the strings in midfield and City struggled to cope with the runs of Van Persie and Nasri.
Only poor finishing and a couple of fine saves from Ben Foster kept the scoreline respectable.
Birmingham had already slipped back into the relegation zone before kick-off and failed to match the quality of their performance in the midweek draw with Manchester United.