The club launched the internal probe earlier this week after it was alleged Cook made light of cancer sufferer Dr Anthonia Onuoha's condition. Cook had denied he sent the message and claimed his email had been hacked, possibly by a rogue employee.
The club, however, have now concluded "there is foundation to Dr Onuoha's allegations" and Cook has admitted to an "error of judgement". Cook, whose tenure has been blighted by several public relations gaffes, said: "I am privileged to have held my position at Manchester City Football Club and to have experienced the opportunities that it has presented."
He added: "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."
The offending email, which was sent 11 months ago, only came to light this week. The message had apparently been intended for City's football administrator Brian Marwood but been sent to Dr Onuoha by mistake.
Dr Onouha, who was representing her son at the time and was involved in a contract dispute with the club, had emailed Cook prior to that. She had written that while she was "ravaged with cancer and ongoing chemotherapy" she was still able to fight on her son's behalf.
The reply she received, addressed to 'Brian', read: "Ravaged with it! I don't now how you sleep at night. You used to be such a nice man when I worked with you at Nike." Dr Onuoha this week said: "When I opened up my emails and saw the message, it was the worst day of my life. Even worse than being diagnosed with cancer."
Cook apologised on behalf of the club to Dr Onuoha for the email but denied having sent it. He said an investigation was being undertaken and that one employee was facing disciplinary action. However, after further evidence emerged suggesting the email had originated from Cook's iPad, the board looked into the matter.
City's statement read: "Manchester City Football Club announce that the board-led review initiated on Monday of this week, as a response to allegations made to the football club, football authorities and a national newspaper by Dr Anthonia Onuoha, has been concluded.
"As a result, 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."
Source: PA
Source: PA