The striker was rescued by Coventry last year on his release from a prison sentence for sexual assault and last week told the club he would be happy to sign a two-year contract.
King swings it: Marlon King will join Birmingham from Coventry
However, Birmingham stepped in to offer a longer deal, which would see the 31-year-old secure until 2014. King had his medical yesterday and will complete the formalities.
Birmingham, who have rejected a ?5million bid for Cameron Jerome from Bolton, face a fans' backlash too with message boards overflowing with criticism.
Blues' boss Alex McLeish has already signed Cardiff winger Chris Burke as he builds a squad aimed at a quick return to the Barclays Premier League following relegation last season.
But his move for King has provoked a furious reaction from Coventry chairman Ken Dulieu who believed the player had verbally committed to a new contract.
Rebuilding job: McLeish is charged with taking Birmingham back up
Dulieu said: 'We are fuming about this and feel we have been betrayed. We had reached agreement with Marlon and his advisor and verbally everything was settled and agreed, and now we find out that a deal has been done with Birmingham.
'The club and our supporters have fully backed Marlon on his return to the game and I think we deserve better than this.'
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Explore more:People: Marlon King, Alex McLeish Places: Birmingham, Cardiff