With hat-trick hero Dimitar Berbatovleading the way, United were rampant against opponents who are inmeltdown ahead of their big Carling Cup semi-final on Wednesday.
Sir Alex Ferguson's class of 2011remain unbeaten for the season and on the verge of breaking a proudrecord established by their 1999 Treble winners.
Close up: Dimitar Berbatov scores the opening goal for Manchester United
Berbatov's hat-trick, accompaniedwith a perfect team goal converted by Ryan Giggs and a solo effort fromNani, left the Premier League leaders convinced another title is on itsway.
The only cloud on the horizon wasanother blank for Wayne Rooney. He played well enough and offered threeassists but he will still feel sore after missing a sitter to leaveBerbatov with all the glory.
Even Sir Alex Ferguson, not a man who usually overdoes sympathy, said: 'We hope the goals will come. He deserves it.'
But Rooney's problems in front of goal are doing nothing to stem the United tide.
Yesterday's win stretched theirunbeaten league run to 27, just a couple of games short of the clubrecord set by David Beckham's boys.
With Blackpool, Aston Villa at home,and Wolves coming up, you would be a brave man to bet against thembeating it, even if emulating Arsenal's Invincibles and going the wholeseason unbeaten will be harder.
Berbatov has scored 10 goals in hislast four league games at Old Trafford. And his third hat-trick of theseason was always on the cards once Birmingham's defenders went tosleep from a corner after 95 seconds.
As Ferguson said: 'Berbatov wasmarvellous and could have scored more. We were a bit wasteful at timesbut we can't complain with the scoreline. It helps the goal difference.We have some tough games ahead but hopefully the players can grasp thenettle.'
Fine finish: Ryan Giggs drills the ball past keeper Ben Foster
Ferguson always beats his formerplayers and a bedraggled Alex McLeish - who was in Fergie's team atAberdeen - was fortunate his side got away with conceding just five.
Birmingham's Chinese owners are making things as difficult as they can for their manager.
They desperately need a striker but McLeish has not been backed in his efforts to sign Kenny Miller and Robbie Keane.
Others want out at St Andrew's andBirmingham fans chanted 'You're not fit to wear the shirt' whenBlackburn Rovers target Seb Larsson came on as a substitute.
McLeish summed up the mood bysaying: 'To lose the first goal like that was criminal. The second goalwas verging on the amateur. Judging by the feedback in the dressingroom, the players were angry more than anything by the result.'
United do not normally pump the ballin the air, but with Ferguson sensing a weakness in Birmingham keeperBen Foster, who was at United until the summer, he packed his team withsix-footers like John O'Shea, Dimitar Berbatov, Nemanja Vidic and ChrisSmalling.
The pay-off came early when O'Shea headed a Giggs corner goalwards and Berbatov prodded it over the line from close range.
Berbatov was foiled by Foster when clean through shortly afterwards but still grabbed his second on the half-hour.
Roger Johnson gave the ball awaycheaply to Anderson, he transferred it to Rooney and the England manreleased Berbatov, who cut inside and finished with a crisp low drive.
In his face: Birmingham keeper Ben Foster leaps to block the header of Chris Smalling
The killer goal arrived on the stroke of half-time and involved lovely build-up work between Rooney and Berbatov before the England man swept a low cross for 37-year-old Giggs to score.
'It was an exciting goal, great interchange play,' said Ferguson.
The only surprise about the second half was that United only added two more goals.
That was partly Rooney's fault for heading wide from three yards after Nani had set him up on a plate.
He still has not scored at Old Trafford in open play since March and has just two league goals, both penalties, at home for United this season. It does not really matter as long as Berbatov continues scoring.
His hat-trick goal arrived after Rooney had brilliantly controlled a punt from Edwin van der Sar, released Giggs and allowed the Welshman to cross for Berbatov in the six-yard box.
Ferguson had the luxury of giving Patrice Evra and Giggs an early rest. Rooney unselfishly trotted over to the left wing to allow Michael Owen a rare chance up front alongside Berbatov.
Keith Fahey missed from even closer than Rooney had done after Birmingham debutant David Bentley set him up, but it was United who scored again - Nani firing in with his umpteenth shot of the afternoon.
'Eventually,' smiled Ferguson. 'We could have scored more but I can't complain at the result.'
Sign me up, Fergie! United veteran Giggs wants new deal at Old Trafford Wantaway Larsson being forced to stay at Birmingham claims his furious agentAll the latest Birmingham City news, features and opinionAll the latest Manchester United news, features and opinion
Explore more:People: Dimitar Berbatov, Ryan Giggs, Nani, Alex Ferguson, Cristiano Ronaldo, Ian Holloway, Michael Owen, John O'Shea, Roger Johnson Places: Birmingham, Liverpool, United Kingdom