Julian Gray headed Birmingham into the fifth round of the FA Cup after Steve Bruce's side took advantage of promotion-chasing Reading resting most of their first-team players.

Mikael Forssell had bundled in an opener on the half-hour mark at St Andrews, but Stephen Hunt levelled for the Coca-Cola Championship leaders with a spectacular volley.

With extra-time to settle the replay looming, Gray, hitherto a peripheral figure, headed home from close range after Jiri Jarosik had found him with a deft cross.

While Reading can now concentrate on securing promotion to the top flight, Birmingham now have the distraction of a cup run as they battle against relegation.

Steve Coppell's selection suggested he had an eye on Championship deeds, but he still retained an impressive spine of a team with Ibrahima Sonko, Steve Sidwell and Dave Kitson providing genuine quality.

Kitson, with 17 goals already this season, wasted no time in getting into the thick of the action, attempting to take advantage of leading the attack in the absence of rested Leroy Lita and Kevin Doyle.

After six minutes Kitson should have opened the scoring when Birmingham's defence was split by Brynjar Gunnarsson.

The midfielder escaped two challenges in the centre circle before perfectly weighting his pass to Kitson, whose shot was just the wrong side of Maik Taylor's post.

But the early chance was not an indication of things to come, with the unfamiliar line-ups struggling for cohesion in a scrappy opening where possession switched frequently and chances proved difficult to create.

Inexperienced centre-back duo Alex Bruce - Blues boss Steve Bruce's son - and Marcos Painter limited the threat of the visitors and allowed Birmingham to establish a foothold in the untidy encounter.

Jermaine Pennant offered pace on the right flank, forging an opportunity with 14 minutes gone through his trickery, foxing two opponents before setting up Jamie Clapham to drill a 20-yard effort wide.

Bruce was fortunate to escape a booking a minute later when he clumsily clattered into Kitson, with the striker glancing a header wide from the resulting set piece.

And Birmingham were ahead in the 30th minute through Forssell, who showed perseverance to score with his side's first real chance.

The Finland striker received the ball on the edge of the area before bundling past Ivar Ingimarsson and Sonko. Graham Stack saved the initial effort but Forssell tapped in the rebound after the ball had rebounded off John Halls.

The visitors offered an immediate riposte and should have been level a minute later when Sidwell crashed a volley off the base of the post, with Taylor defying Kitson with a stunning reflex save from the rebound.

Coppell's men appeared to be spurred on by going behind, with Kitson volleying over with 10 minutes remaining in the half, and they were level within five minutes of the restart.

Reading had already threatened through John Oster's dangerous cross after 48 minutes before Stephen Hunt restored parity.

Hunt chased a hopeful punt forward but took advantage when Bruce misjudged the bounce, the midfielder nodding past Mario Melchiot and crashing in his volley off the crossbar.

Jarosik, who consistently tried his luck from long distance at every opportunity, could have restored the lead after 55 minutes when Pennant played him through, but the Czech Republic international saw his effort saved by Stack.

Gray, who had struggled to impose himself on the game, then made his impact. On the hour mark he blazed a volley over after a Pennant free-kick but atoned for his earlier miss by heading in after 66 minutes.

Jarosik had found room on the right before checking back to cross and Gray applied the simple finish from close range.

Coppell responded by bringing on Simon Cox for Long, but the Championship leaders appeared to run out of steam without their first-team regulars.

Cox did have a drive deflected wide, but Reading struggled to create clear-cut openings.

Ingimarsson was booked in the final stages for dissent as Birmingham held on to win.

Teams

Birmingham: Maik Taylor, Melchiot, Bruce, Painter, Gray, Pennant, Clemence (Oji 90), Clapham, Birley (Sadler 59), Forssell, Jarosik.

Subs Not Used: Vaesen, Alsop, Allen.

Goals: Forssell 30, Gray 67.

Reading: Stack, Halls, Sonko, Ingimarsson, Makin, Oster, Sidwell, Gunnarsson, Hunt, Kitson, Long (Cox 68).

Subs Not Used: Federici, Golbourne, Osano, Sinnott.

Booked: Gunnarsson.

Goals: Hunt 51.

Att: 16,644

Ref: P Walton (Northamptonshire).