Vitoria from Portugal, Ukraine side Metalurg Donetsk, Skonto of Latvia, Poland's Slask Wroclaw and Romanian duo Pandurii Targu Jiu and Petrolul Ploiesti have all been involved in European competition this season.
UEFA said that, as a result of its assessment of the financial status of participating clubs this season, it had requested further information in the case of 31 clubs, with prize money temporarily withheld in the case of six.
"The UEFA Club Financial Control Body (CFCB) investigatory chamber, chaired by Jean-Luc Dehaene, has announced that six clubs involved in the 2013/14 UEFA club competitions have been referred to the CFCB adjudicatory chamber," UEFA said in a statement, adding that a verdict is expected in all six cases before the end of the year.
"Following further investigations and based on new information and evidence provided by the clubs, the CFCB investigatory chamber decided to close the cases involving 25 clubs.
While the overall impact of the financial fair play rules continues to be debated, UEFA has claimed them to be a success, pointing to the dramatic fall in outstanding debts held by clubs.
"The introduction of the UEFA Club Licensing and Financial Fair Play Regulations has already had a very positive impact on the scale of overdue payables towards clubs, employees and social/tax authorities, as they have decreased from E57m in June 2011 to E9m in June 2013," UEFA said.
"A similar trend was observed in September 2013."