Manchester City chief executive Garry Cook has resigned over claims he emailed defender Nedum Onuoha's cancer-suffering mother mocking her illness.
Cook offered his resignation after an investigation by City found "there is foundation" to the allegations.
He had denied sending the message, claiming his account was hacked.
The club issued a statement confirming the board had accepted Cook's resignation and that chairman Khaldoon Al-Mubarak had apologised to Dr Onuoha.
Defender Onuoha's mother, who acts as his agent, had previously sent a message to Marwood and Cook, explaining that while she was "ravaged with cancer" it would not prevent her negotiating on behalf of her son.
Dr Onuoha then received an email, allegedly from Cook and addressed "Brian", which apparently ridiculed her use of language in describing her illness.
The email is reported to have read: "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. G".
Dr Onuoha told the Sun the email had left her "humiliated and devastated".
The claims led City's board to launch an investigation and, following its findings, Cook offered his resignation and Khaldoon wrote to Dr Onuoha "to apologise for any distress caused".
In the statement, Khaldoon said: "Garry has made a remarkable contribution to Manchester City Football Club over the past three years. His judgement in this matter should in no way lead to his accomplishments being overlooked.
"On every level, the club is unrecognisable from the organisation which he inherited and our staff and supporter services, community outreach and commercial activity have seen unparalleled growth under his direction with yet more projects to be realised on the horizon.
"On behalf of Sheikh Mansour and the Board, I would like to thank him for his energy and tireless commitment to serving all those connected with the football club. He will always be welcome at Manchester City."
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."
City director John MacBeath will fill the role of temporary chief executive officer while a replacement is sought.