Garry Cook has resigned as chief executive of Manchester City over claims he sent an offensive email to the mother and agent of defender Nedum Onuoha.
It was reported that Cook initially claimed his email had been hacked into but has now admitted to an "error of judgement''.
Dr Anthonia Onuoha emailed City's football administrator, Brian Marwood, last October stating that, despite being "ravaged with cancer", she would continue to hold talks regarding a dispute over her son's contract.
A fortnight later, Dr Onuoha alleged she received an email, seemingly from Cook addressed to Marwood, that said: "Ravaged with it!!......I don't know how you sleep at night. You used to be such a nice man when I worked with you at nike (sic)."
Dr Onuoha replied the following day, writing: "Thanks very much for your insightful e-mail." Cook has since denied sending the email, citing that his account had been hacked while he was on holiday in South Africa.
However, an investigation by City, initiated on Monday, concluded that the allegations against Cook did have substance and the chief executive tendered his resignation on Friday.
City said in a statement: "The Club can confirm that there is foundation to Dr Onuoha's allegations and the Chairman has written to apologise to Dr Onuoha for any distress caused.
Following the findings of the review, Chief Executive Officer Garry Cook has offered his resignation, which has been accepted with regret by the Board."
Cook said: "I am privileged to have held my position at Manchester City Football Club and to have experienced the opportunities that it has presented. The privilege is in part offset however by the significant personal focus which has at times, detracted from the magnificent achievements of those working at the football club.
"It is that factor, together with my error of judgement in this matter that has prompted me to reach this decision, which I believe is in the best interests of the football club.
"I wish Roberto and his team and all of the fans and supportive partners of the Club the very best for exciting times ahead. I would also like to put on record my unreserved thanks to Sheikh Mansour, Khaldoon Al Mubarak and the Board and all of the very talented staff at the Club for their support throughout my tenure at Manchester City. I know the Club will go from strength to strength."