Birmingham suffered their heaviest defeat of the season as Manchester United hammered them at Old Trafford.

Dimitar Berbatov scored a hat-trick, while Ryan Giggs and Nani added a further goal each against a lifeless Blues side, who appeared to have their minds firmly on Wednesday’s Carling Cup semi-final second leg against West Ham.

Probably with the next game in mind, manager Alex McLeish opted to rest first team regulars Cameron Jerome, Sebastian Larsson and Craig Gardner, in favour of striker Matt Derbyshire, new signing David Bentley and, surprisingly, 19-year old Jordon Mutch.

Blues suffered the worst possible start when Berbatov flicked home Giggs corner to open the scoring after less than two minutes.

The visitors appeared shell-shocked and goalkeeper Ben Foster almost gifted the Bulgarian a second goal just three minutes later, when his awful clearance went straight to Berbatov 25 yards from goal.

But the former Manchester United goalkeeper reminded his old boss why he was held in such high regard at Old Trafford, by saving the striker’s low shot with his feet.

The visitors continued to gift the home side too much space and Nani should have hit the target with his curling strike from the edge of the box on 19 minutes, but dragged the ball narrowly over.

Not that it mattered as Berbatov doubled the home side’s lead 11 minutes later, after Roger Johnson gave the ball away in midfield.

The hosts ruthlessly launched a counter-attack and Rooney found Berbatov, who smashed the ball home after a winding run.

Foster then saved from Darron Gibson and Mutch deflected Giggs’ low shot just wide, as United poured forward looking for a third.

But the game was effectively over as a contest in the first minute of first-half stoppage time when Giggs slotted home Rooney’s superb cross, albeit under nowhere near enough pressure from the Blues defence.

United manager Sir Alex Ferguson swapped Patrice Evra for the less experienced Fabio Da Silva at half-time, while McLeish brought on a second striker, swapping Alexender Hleb for Cameron Jerome.

It did nothing to stem the tide, however, as Rooney should have made it four within three minutes of the restart.

Somehow the England star nodded wide from just a few yards out but Manchester United could afford to miss chances, such was their dominance.

Berbatov completed his hat-trick five minutes later, stabbing home Giggs’ cross from another scything counter attack.

The gulf in class was emphasised when Giggs was replaced by Michael Owen at this point but, even so, this was far too easy for the home side.

Keith Fahey then missed a glorious chance to pull back a consolation goal, somehow stabbing the ball wide from a few yards out, after Derbyshire’s cross had beaten goalkeeper Edwin van der Sar.

Nani was being given free range to shoot as Blues defenders looked terrified to tackle the Portuguese winger, who missed the target on several occasions before finally getting a goal 14 minutes from time.

The finish, from outside the box, was superb but again he was allowed to run and effectively pick his spot, as the Blues’ back line backed off him.

Craig Gardner replaced a tired-looking Bentley with six minutes left and the final minutes were played out at training ground pace with the result in no doubt.