The mother and agent of the Manchester City defender Nedum Onuoha has sent official letters of complaint to the Football Association and Premier League after receiving an offensive email, during her year-long fight against cancer, which appeared to have been sent from the account of Garry Cook, the club's chief executive.

Cook has denied responsibility for the message, claiming that a member of City's staff had hacked into his account and sent the email as a joke. He said an internal inquiry had been opened.

Dr Anthonia Onuoha received the email, which was intended for Brian Marwood, City's football administrator, in regard to a contract dispute her son was having with the Eastlands club. Dr Anthonia 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.

She then received an email from the club addressed "Brian", which stated: "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: "When I opened my emails and saw the message, it was the worst day of my life, even worse than being diagnosed with cancer. I couldn't understand how anybody could behave like that. I just cried and cried for hours. I'm critically ill and at that point I was undergoing chemotherapy. I was just so shocked but I couldn't tell Nedum or any of my family because I didn't know how they would react."

Dr Onuoha said she replied to Cook the following day, writing: "Thanks very much for your insightful e-mail."

In his response Cook denied sending the message. Dr Onuoha said: "When Cook replied to me, he said that he was confused, that he'd been away on holiday in South Africa and that somebody had hacked into his email account and whoever sent the email to Brian Marwood meant it as a practical joke. It was all very stressful at the time, in fact it still is.

"The illness I have is very stressful, I'm having chemotherapy and radiotherapy, including having radiotherapy on my brain and this year has been traumatic. I didn't want anybody adding to my stress but that is what they've done

"The feeling of humiliation still persists. Now I just want the FA and the Premier League to see what's happened and to take the appropriate action. Treating people like this is completely abominable."

Cook apologised for "any hurt or embarrassment this may have caused" as well as denying sending the email.

He told Dr Onuoha: "I cannot and will not condone this unacceptable behaviour. I can confirm Brian did not receive this email and disciplinary action is currently underway regarding one of our employees."

Dr Onuoha's son came through the ranks at City and has started more than 70 league games for the club. He spent a successful period on loan at Sunderland last season. His mother has represented him throughout his career.