UEFA has revealed that 76 clubs in European competition this season - about one third of the total - are being investigated for possible breaches of its financial fair play (FFP) rules.
The 76 clubs all failed UEFA's break-even calculations for 2012 and have been asked to provide financial information for 2013. Manchester City, and French club Paris St Germain are believed to be among the 76 clubs involved.
The clubs 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.
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 that are facing possible action. Some clubs will be offered settlements if they agree to certain conditions, but clubs who have been found to have severely breached the conditions will be referred to the adjudicatory arm of the financial control body which will announce sanctions in the middle of June.
The only appeals will be straight to the Court of Arbitration for Sport and UEFA expects final decisions will be made before the Champions League and Europa League group draws in August.
UEFA's legal affairs director Alasdair Bell is anticipating a number of legal challenges.
He said: "This will be no surprise to us. We are not afraid of them being contested. We fully anticipate there will be challenges - it would be strange if there weren't. July and August could be a busy time."
Chelsea are understood not to be one of the 76 clubs facing investigation by UEFA. The Blues made a £49.4million loss last year but actually made a £1.4million profit in 2012 so are set to escape any action.
Among the English clubs, Manchester City would have the most to fear having returned losses of £97.9million in 2012 and £51.6million last year.
A club's annual figures do not directly reflect UEFA's break-even calculations but are an indication of those that are at risk - for example spending on facilities, youth development and contracts signed before 2010 are not included.
UEFA's rules state clubs can be up to 45million euros (£37million) in the red over 2012 and 2013 so long as the owners put in equity to cover the losses. If clubs are over the 45million euro mark over the two years then they will face sanctions.
Infantino said the rules were necessary to "protect European football from greed, reckless spending and financial insanity".
Earlier this month Karl-Heinz Rummenigge, chairman of the European Clubs' Association, said he "cannot imagine that Paris St Germain are complying with financial fair play".
Qatari-owned PSG has announced a huge, back-dated sponsorship deal with the Qatar Tourist Authority.
UEFA refused to discuss individual clubs, but said all sponsorship deals with related parties to club owners would be assessed to ensure they are of fair value.