Garry Cook (R) with Manchester City's chairman Khaldoon al-Mubarak who has heavily invested in club [GALLO/GETTY]

Manchester City chief executive Garry Cook resigned on Friday amid a storm over an email in which he appeared to deride the cancer-stricken mother of a player, a statement said.

Cook stepped down from his post with the Premier League club following an internal investigation into the circumstances surrounding an email sent to Dr Anthonia Onuoha, the agent and mother of City player Nedum Onuoha.

Cook had initially denied that he sent the email, claiming someone hacked into his email account, but in a statement following his resignation on Friday, Cook admitted to an "error of judgement".

"I am privileged to have held my position at Manchester City Football Club and to have experienced the opportunities that it has presented," Cook said.

"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."

Accepted with regret

A statement from City said an internal investigation had found there was "foundation" to Dr Onuoha's allegations.

"Following the findings of the review, Chief Executive Officer Garry Cook has offered his resignation, which has been accepted with regret by the Board," said the statement.

According to reports, Dr Onuoha wrote to City official Brian Marwood about a contract dispute between the club and her son, who is an England Under-21 international.

In an email sent to Marwood last October, she said that, although her body was "ravaged with cancer and ongoing chemotherapy", she was still able to fight her son's corner.

Two weeks later she received an email, apparently written by Cook but destined for Marwood and sent to Dr Onuoha by mistake.

The Sun newspaper claimed: "The e-mail was addressed 'Brian' then said: '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 (sic).'"

A former executive at sportswear giant Nike, Cook was the subject of negative headlines in the early days of his tenure at City.

He once described controversial former City owner Thaksin Shinawatra as "a great guy to play golf with," and said an attempt to sign Brazilian superstar Kaka failed because the player "bottled it".