leading clubs.
In a statement, ECA chairman Karl-Heinz Rummenigge echoed their claims.
"As the Olympic Games are not included in the international calendar, the obligation to release players for national team matches in accordance with the FIFA rules don't apply," the statement said.
"Therefore, I support all those clubs who are currently at odds with the idea of losing important players."
In comments published earlier on the Bayern Munich website, Rummenigge - also president of the German champions - claimed to have come away from talks with Blatter in March with the message that releasing players for the games was not mandatory.
"Then the clear message was: The Olympics is not listed in FIFA's official tournament calendar, which means there is no duty to release players," he said.
Rummenigge backed the German clubs' plans to take their case to the CAS.
Caught in the middle of the row are the players. Diego described his decision to leave Werder without permission and fly to Paris to join up with the Brazil squad on Tuesday as "one of the most difficult" of his career.
Diego added that it was never his intention to disobey his employers, but he feared that he would jeopardise his international future if he did not respond to Brazil's call.
"I was under a lot of pressure since I knew that my future in the Brazil national team could be in danger if I did not travel to the Olympics," he said.
"That would have made me very unhappy and could not be Werder's aim at all.
"I just hope that all fans, particularly those of Werder, can understand me at least a little."
The day did end with an amicable solution for some, however, as Hamburg reached a compromise with defender Vincent Kompany and the Belgian Football Association, allowing the 22-year-old to play at the Olympics.
The deal will see Kompany divide his time between Belgium and Hamburg over the next few weeks, skipping most of Belgium's preparations to stay with Hamburg, but joining the national team in time for their group fixtures in Beijing.