Manchester City are believed to be among 76 clubs in European competition this season – about one third of the total – being investigated for possible breaches of Uefa's financial fair play rules. Paris Saint-Germain are also thought to be involved.
The 76 clubs all failed Uefa's break-even calculations for 2012 and have been asked to provide financial information for 2013.
They will all now have their finances assessed by the club financial control body to see if the breaches have continued and whether sanctions should apply.
The Uefa general secretary Gianni Infantino said: "This figure of 76 clubs is a high figure but it has to be looked at in the perspective of what the end figure will be."
Clubs face a range of sanctions ranging from a warning or a fine up to being forced to play in Europe with a salary cap on the squad, or even being barred from competing and having trophies stripped.
The sanctions will apply from the start of next season but in April Uefa will name the clubs who are facing possible action.
Among English clubs, Manchester City would have the most to fear having returned losses of £97.9m in 2012 and £51.6m last year.
Chelsea made a £49.4m loss last year but actually made a £1.4m profit in 2012 so may escape any action.