Manchester City were on Monday night holding top-level talks to consider Garry Cook's position after the offensive email that has prompted senior figures in Abu Dhabi to demand an explanation from the chief executive.

City have consulted their legal department as the controversy intensifies, with the growing sense that the club's owner, Sheikh Mansour, and chairman, Khaldoon al-Mubarak, are not willing to tolerate what has happened and are considering all options.

After a day of near-silence from Eastlands City released a brief statement last night, more than 24 hours since being informed of the Sun's intention to run a story about Nedum Onuoha's mother having received an email mocking her fight against cancer. "The club can confirm that a board-led review has been launched into the matter," the statement said.

Dr Anthonia Onuoha, who also works as her son's agent, had notified Cook and the club's football administrator, Brian Marwood, during previous correspondence that she was "ravaged" with cancer. She was then copied into an email which had been sent from Cook's account to Marwood. It was addressed "Brian" and 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"

The email was sent 11 months ago from an iPad but it is only now that Dr Onuoha has been persuaded to publicise it. Nonetheless it is threatening to have serious ramifications for a man who earns £1.8m a year at Eastlands and has played a significant part in the club's transition under the Abu Dhabi United Group's ownership.

Cook denied responsibility when Dr Onuoha let him know she had received the "insightful email" and went on to claim that his account had been hacked and that an internal inquiry had been opened. He said the culprit had been identified and disciplinary proceedings had begun.

Sheikh Mansour and Mubarak want to know if he is sticking by that story and if it can be corroborated. Sensitive to any kind of bad publicity, they have remained behind Cook during previous controversies but regard this one as by far the most damaging and serious.

The investigative process was held up because Cook is in Portland, Oregon, visiting his wife and children. Various City officials believe Cook to be in a vulnerable position even though he has previously had a strong relationship with the men who have turned City into the most financially powerful club in the world.

Cook will at least be spared any disciplinary action from the Football Association and the Premier League, both of whom are reluctant to get involved despite Dr Onuoha asking them to take action.

She said: "The illness is very stressful. I'm having chemotherapy and radiotherapy 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," she added. "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 had apologised for "any hurt or embarrassment this may have caused" and reputedly told Dr Onuoha: "I cannot and will not condone this unacceptable behaviour. Brian did not receive this email and disciplinary action is under way regarding one of our employees."

Onuoha, a 24-year-old defender who came through the club's academy, has been told he does not figure in the plans of the City manager, Roberto Mancini. A loan move had been arranged to Everton but fell through because City refused to pay any part of his £38,000-a-week salary.

Onuoha has links with Kia Joorabchian, the controversial businessman who represents Carlos Tevez and who once counted among Cook's allies. The two men have since fallen out and City were so dismayed when they found out Onuoha had become involved with Joorabchian they are understood to have notified the FA on the basis that he does not own a Fifa licence to work as an agent.