BIRMINGHAM City made their Premier League status count by inflicting a heavy FA Cup third round defeat on Millwall at The Den.
Matt Derbyshire (2), David Murphy and Cameron Jerome all found the target for the Bluenoses, while Danny Schofield grabbed a late consolation in stoppage time.
Manager Kenny Jackett made a trio of changes to the side which drew 0-0 at Derby on Monday.
Paul Robinson partnered Danny Shittu in central defence, while Neil Harris was handed a rare start in place of Liam Trotter.
Millwall had the first opportunity in the fourth minute when Steve Morison broke clear down the left and found James Henry, whose shot was put behind for corner a Birmingham defender.
Henry went even close one minute later, unleashing a powerful shot from 25 yards which just flew past Maik Taylor's post.
Danny Shittu placed a 10th minute header straight at Taylor after rising high to get on the end of Henry's corner.
Morison really should have put the Lions in front when he broke clear on the quarter hour to get on the end of Jack Smith's pass but instead placed his low shot wide of the target.
He was made to rue the miss even more when Birmingham broke the deadlock seconds later with their first chance of the game.
Jean Beausejour's low pass found Derbyshire, who rounded Forde before firing home an exquisite chip from just inside the area.
Henry's shot on the turn from 25 yards lacked power and was never going to trouble Taylor as the Lions tried to respond to the setback.
Birmingham claimed they should have had a penalty on 26 minutes after Tony Craig upended Derbyshire inches outside of the area.
Nevertheless, Murphy stepped up to take the free-kick and placed his effort over the Millwall wall and beyond the reach of Forde to make it 2-0 to the visitors.
Derbyshire almost added a third when a Jordon Mutch free-kick fell to him in the area but this time Forde produced a save.
Morison was unlucky at the other end to see his overhead kick go agonisingly wide after Taylor had failed to deal with a cross, while Henry fired over from long range and Robinson had a header blocked as Millwall fought back.
Morison actually had the ball in the net four minutes before half-time but his effort was cancelled out as he was adjudged to have been in an offside position when connecting with Henry's cross.
But there was more woe for the Lions on the stroke of the break as a clever passing move ended with Derbyshire lifting a shot over Forde to make it three for the visitors.
Henry's 48th minute attempted chip of Taylor sailed harmlessly over the bar but Millwall were handed a lifeline when Scott Dann pushed over Hazrris.
Morison stepped up to take the 51st minute spot kick but his tame effort was easily saved by Taylor, perhaps a sign this just wasn't meant to be Millwall's day.
That feeling was reinforced moments later when Henry's goal-bound shot was blocked by ? before Harris blazed the loose ball over the bar.
Jackett made a double substitution on the hour by throwing on Schofield and Kevin Lisbie for Craig and Harris.
Beausejour's 62nd minute effort was deflected onto the bar, while Lisbie fired over from six yards after being set up by Smith.
Substitute Jerome piled on the misery for the Lions by heading home Keith Fahey's inviting 72nd minute cross.
It should have been five if Beausejour had passed to the unmarked Jerome instead of going for goal himself with a shot that was comfortably held by Forde.
Gardner could just as easily got a fifth but his powerful shot drifted inches wide and Tamika Mkandawire headed wide as Millwall attempted to at least get a consolation.
But Birmingham always looked dangerous going forward as Fahey fired an 83rd minute volley straight at Forde.
Schofield produced a decent finish by firing a shot past Taylor to at last get the hosts on the scoresheet in stoppage time.
But the damage had long since been done as Birmingham marched on to the fourth round.
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Millwall: Forde, Smith, Robinson, Shittu (Ward 73), Craig (Schofield 60), Henry, Mkandawire, Abdou, Barron, Harris (Lisbie 60), Morison. Subs not used: Mildenhall, Grabban, Hackett, Marquis. Att: 9,841.
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