Millwall were blown away inside half an hour as Birmingham cruised to a 3-0 win at St Andrew’s.
The Lions were behind before they had even settled, with Alex Cochrane firing a left-footed shot from outside the box into the bottom corner from a set piece inside the first minute.
It got worse on 16 minutes when Max Bird finished from the centre of the box after a headed pass from Jhon Solís.
Bird then turned provider for Jay Stansfield, who made it 3-0 on 29 minutes.
Millwall goalkeeper Max Crocombe at least kept the scoreline respectable, saving from Stansfield and Solís in the first half and twice more after the break.
The visitors’ best chances came from Caleb Taylor, whose header was saved by Ryan Allsop early on, and Camiel Neghli, who smacked the bar with a left-footed strike from distance in the 75th minute.
Josh Coburn and Romain Esse were sent on with 20 minutes to go, but there was no way back in front of a crowd of 25,044.
It was a chastening night for Millwall after Saturday’s 4-1 win over Derby had lifted spirits at The Den.
Birmingham, meanwhile, remain firmly in the promotion picture after another emphatic display on home soil.