Coventry manager Andy Thorn feels 'betrayed, staggered and dismayed' by Marlon King's decision to sign for Birmingham.
Former Sky Blues boss Aidy Boothroyd offered King a route back into the game after he was released from prison last July, signing for the club until the end of the season.
A new deal was offered which King verbally accepted last week, although the 31-year-old alleges Coventry's hesitation in sending across a written contract allowed Blues to make their move.
But Thorn has dismissed the forward's version of events, stating there was only 24 hours between him getting the nod from King and then hearing he was in talks at St Andrew's.
Thorn told the Coventry Telegraph: "I feel betrayed on a personal and professional level by what has happened. I have no idea what sort of advice he has been given but I think he should have considered how well he has done here, the way the supporters have backed him and accepted him.
"What Marlon is claiming simply does not stack up. He spoke to me last Thursday to confirm he wanted to stay, the story came out in the media that he had agreed to stay with us, yet 24 hours later we heard rumours that he was talking to Birmingham. I am staggered and dismayed at what has happened."
Coventry chairman Ken Dulieu has echoed Thorn's feelings and was similarly scathing about their former striker
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. We have shown great faith in him, helped him rebuild his career and were delighted that he was due to continue playing for us. To hear that he has now changed his mind and gone back on what had been agreed is very hard to take.
"News of his decision to sign a new contract with us leaked out, and that is why we had to publicly confirm the verbal agreement. We would have preferred all the formalities to be completed but we had no choice. I really feel for supporters."
King himself countered: "I haven't done anything wrong. Yes, I did phone the manager because I was happy with the three-year deal and I was willing to sign. I said let's do it, I want to sign.
"I said get the contract over to me but that didn't materialise. I called my advisor, Tony Finnigan, and asked what was going on and he said nothing had come through on the computer in writing."