The focus of the investigation is a number of alleged breaches of league rules on how clubs finance their activities, the so-called Financial Fair Play.
The German magazine Der Spiegel has published leaked e-mails claiming that Sheikh Mansour, the club owner, in different ways puts more money in the club than what the framework allows.
Manchester City has in the past dismissed the allegations as substanslösa, but Uefa want to take a closer look at how it relates. Would the City be held guilty of infringement, it may mean that the club closes from games in the european cups.