Cameron Jerome stepped off the bench to fire in a late winner at Watford and put Birmingham in pole position to join neighbours Wolves in winning promotion.
Jerome came on for Garry O'Connor with 20 minutes remaining and smashed a shot into the top corner to give Alex McLeish's side a vital three points.
Watford goalkeeper Scott Loach had an inspired first half to frustrate the Blues, Lee Bowyer going closest.
Watford's best chance fell to Grzegorz Rasiak, but his header was well saved.
Birmingham are now four points clear of third-placed Sheffield United - who must face play-off chasing Burnley at Turf Moor on Monday.
Watford boss Brendan Rodgers:
"In the first half it was difficult for us to retain possession up front and we never got going.
"They know they have had a tough game but they are back on the bus with a win and that is the sign of a good side.
"When you lose to a deflected goal that wasn't going in it makes it even worse."
Birmingham manager Alex McLeish: "This win gives us a platform and keeps us in the driving seat.
"It puts the pressure on teams below us to get a result.
"I know how Sheffield United will be feeling. It is tough to respond to pressure and the more you handle the mental side of the game the better you will perform.
"But we have a tough task next week. We have to play Preston who had an amazing result today by beating Cardiff 6-0 and they will come at us with full confidence."
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