Uefa has imposed its first financial fair play sanctions by temporarily withholding prize money from 23 clubs for failing to comply with the rules.
Europa League winners Atletico Madrid, Malaga, Sporting and Fenerbahce are among those under scrutiny.
Clubs face further investigation because of overdue payments to other clubs, employees or tax authorities.
European football's governing body introduced the rules to force clubs to operate within their means.
The 23 clubs have been given a deadline of 30 September to prove that their finances are in order.
Teams playing in all Uefa club competitions have been monitored since the start of the 2011/12 season.
The are tasked with breaking even in 2012 and 2013 and being assessed during the 2013/14 season.
Clubs under investigation:
FK Borac Banja Luka (Bosnia)
FK Sarajevo (Bosnia)
FK Zeljeznicar (Bosnia)
PFC CSKA Sofia (Bulgaria)
HNK Hajduk Split (Croatia)
NK Osijek (Croatia)
Club Atletico de Madrid (Spain)
Malaga CF (Spain)
Maccabi Netanya FC (Israel)
FK Shkendija 79 (Macedonia)
Floriana FC (Malta)
FK Buducnost Podgorica (Montenegro)
FK Rudar Pjevlja (Montenegro)
Ruch Chorzow (Poland)
Sporting Clube de Portugal (Portugal)
FC Dinamo Bucuresti (Romania)
FC Rapid Bucuresti (Romania)
FC Vaslui (Romania)
FC Rubin Kazan (Russia)
FK Partizan (Serbia)
FK Vojvodina (Serbia)
Fenerbahce SK (Turkey)