Birmingham failed to capitalise on their early superiority and Reading responded with a series of swift breaks. Garath McCleary released Pavel Pogrebnyak inside the Birmingham area but the Russian's fierce shot was well saved by Darren Randolph.
Reading's pressure finally paid off in the 32nd minute as they took the lead, albeit with a goal that was tinged with good fortune.
Chris Burke was penalised for handball and Guthrie's free-kick from wide on the left did not appear to carry too much danger. Yet it somehow found its way through a mass of bodies in the area and nestled in the far corner of Randolph's net.
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Southend United are promoted to League One after a nail-biting play-off final against Wycombe, which they won 7-6 on penalties.
Kate Haskell has the weekend forecast for the east of the region
Kate Haskell has the weekend forecast for the west of the region.