* Atletico, Malaga and Sporting among those affected
* Clubs not taking rules seriously, says Rummenigge (adds ECA comments, Malaga statement)
Sept 11 Europa League winners Atletico Madrid were among 23 clubs to have prize money withheld as European soccer's governing body UEFA revealed the first sanctions under its financial fair play rules on Tuesday.
The measures were announced as members of the European Clubs Association (ECA), which represents more than 200 clubs, were warned at a meeting in Geneva that some of them were not taking the new rules seriously.
"ECA members unanimously endorsed the financial fair play project back in 2010," said ECA president Karl-Heinz Rummenigge, who is also Bayern Munich's chief executive.
"However, it seems that quite a few clubs have not understood the message. Time has come to take the new rules seriously. ECA will continue to support financial fair play."
ECA also heard from former Belgian Prime Minister Jean-Luc Dehaene, who heads the UEFA committee charged with enforcing the new financial rules.
"I am still very worried about the current situation," he told clubs, according to an ECA statement.
"The Financial Fair Play Regulations are known for more than two years, but I have the impression that some clubs still need to do their homework."
UEFA said the 23 clubs, who also include Qatari-owned La Liga side Malaga and Portugal's Sporting, were found to have "important" overdue payments to other teams, their own employees or social and tax authorities.
As a result their money for taking part in European competition would be withheld pending further investigation, UEFA said in a statement.
The clubs will have to provide an updated report on Sept. 30 to Dehaene's committee.
"This ... measure will remain in force until all identified balances have been settled in full or until a final decision by the (UEFA committee) is taken," said UEFA.
Among other clubs under investigation are Dinamo Bucharest and Rapid Bucharest of Romania, Turkey's Fenerbahce and Serbia's Partizan Belgrade.
All are involved in this season's UEFA competitions.
There were no clubs from the English Premier League, German Bundesliga, Italy's Serie A or France's Ligue 1 among the clubs named.
Malaga recognised they owed money to the Spanish tax service and said they were trying to reach an settlement.
"One month ago, an internal restructuring process was started to guarantee the future viability of the club," Malaga said in a statement.
"In this process, the club has normalised its financial situation with other clubs, employees and players and has started negotiations with the tax service, with which the club has not reached a final agreement although we have reduced the amount owed considerably in the last month."
UEFA approved the introduction of the far-reaching financial fair play rules in 2009 in a bid to reduce debt and introduce better and more transparent financial dealings among clubs playing in its competitions.
UEFA president Michel Platini said the measures have been introduced to stop clubs from spending their way into oblivion and to force them to live within their means.
They are also intended to stop wealthy owners from trying to buy success with limitless spending on players, distorting the transfer market in the process.
Transfer fees and outgoing payments have been monitored since the start of the 2011/12 season with clubs tasked with breaking even in 2012 and 2013 and being assessed during the 2013/14 season.
If clubs have not come into line by then, they could be thrown out of European competition. (Reporting by Mitch Phillips and Brian Homewood)