Hull's title bid suffered a huge blow at Birmingham as the hosts closed in on the Sky Bet Championship play-offs.
Arsenal loanee Jon Toral's deflected winner handed Blues a 1-0 victory to deny the Tigers top spot.
A draw would have sent Steve Bruce's side above Burnley at the summit but they handed a crucial advantage to the Clarets, who remain a point clear.
Jake Livermore and Michael Dawson hit the woodwork for the Tigers but they stay level on points with third-placed Middlesbrough - who have a game in hand.
Birmingham are now just a point behind the top six after their sixth clean sheet in eight games in an open match where they struck early.
They held out in the face of late pressure after Toral grabbed the winner on 14 minutes.
As it happened
Relive the action from St Andrew's where Hull missed the chance to go top of the Championship.
The 21-year-old, selected ahead of Diego Fabbrini, almost bamboozled himself as he spun past Dawson before regaining control 25 yards out.
His shot initially lacked power and direction but it clipped former Birmingham defender Curtis Davies to fool Allan McGregor and creep in.
It was the first goal Hull had conceded in all competitions in 387 minutes.
The Tigers almost hit back within six minutes when Livermore rattled the post from 25 yards and soon after Dawson's header clipped the top of the bar.
Hull's spirited response curbed any Birmingham threat as the title-chasing Tigers tried to recover.
And they should have levelled after 31 minutes when Mo Diame found Sam Clucas and he ignored handball claims but could only drill at Tomasz Kuszczak with just the keeper to beat.
Sixty seconds later McGregor turned Clayton Donaldson's spectacular overhead kick away to deny the forward a stunning goal.
The chances continued to flow and Toral almost grabbed a second on 37 minutes when his diving header dropped a yard wide.
After the all-action first half, the second struggled to fire until Hull wanted a penalty when Abel Hernandez's shot cannoned off Paul Robinson's arm with 23 minutes left.
Four minutes later Kuszczak pulled off a double save from Robert Snodgrass and Clucas as Hull piled on the pressure.
Michael Morrison then denied Clucas with Birmingham pinned back and Snodgrass shot straight at Kuszczak with three minutes left but Hull failed to find a leveller.