Fellow striker Cameron Jerome had been sent off after just five minutes of his first Blues appearance for violent conduct.

But Danish Under-21 international Bendtner, who was only signed on loan from The Gunners on Thursday, kept his cool after coming on as substitute to drill home a low drive with 12 minutes remaining.

Steve Bruce’s side, installed as favourites to take the title by the bookmakers, were given an early warning that making an instant Premiership return will be far from easy and Colchester deserved a share of the spoils.

Jerome had been introduced as a 65th-minute substitute and the £3million summer signing from Cardiff was quickly given his marching orders by referee Keith Stroud after appearing to catch Colchester skipper Karl Duguid in the face with his arm.

It means he will face an automatic three-game ban.

The summer optimism surrounding St. Andrews after Bruce had revamped his squad looked misplaced for long periods as Colchester were far from overawed and constantly threatened to open up the Birmingham defence.

Birmingham included summer signings Stephen Kelly, Bruno N’Gotty and former Colchester midfielder Neil Danns in their line-up and the early exchanges were played at a frantic pace.

The first decent opportunity fell to Colchester, who had a 4-2 numerical advantage when catching Blues on the counter-attack, but skipper Karl Duguid’s low drive was parried by Maik Taylor.

City retaliated and a corner from Danns was headed on by Campbell to Mikael Forssell who volleyed into the side netting from close range.

A perfectly weighted free-kick from Danns picked out the run of Olivier Tebily but he could only glance his header wide of the far post.

David Dunn was only inches wide with a dipping 25-yard effort as the home side started to take control and Tebily again ventured forward to force Aidan Davison to parry away his shot on the turn.

Forssell came close after a mazy run when his shot flew inches wide of the far post.

Davison blocked a close-range effort from a narrow angle by Campbell but the former Brentford striker was not to be denied and after 30 minutes he put City in front.

A right-wing cross from N’Gotty picked out Campbell and his glancing header gave Davison no chance.

Colchester had an opportunity to respond immediately when George Elokobi’s first time ball found Jamie Cureton in behind the Blues defence, but he hurried his shot and Taylor was not unduly troubled.

Colchester began the second half promisingly and after 51 minutes Richard Garcia brought them back on level terms.

Skipper Karl Duguid’s low cross was only cleared by N’Gotty to Garcia, who this time made no mistake from eight yards out with a rising drive.

Bruce’s immediate response was to send on the on-loan Arsenal duo of Sebastian Larsson and Bendtner in place of Campbell and Clemence, and Jerome then took over from Forssell only to suffer his red-card shame.

But Bendtner raised the spirits of Birmingham and the 24,000-strong crowd’s belief was restored as he kept his nerve before firing past Davison.