Blatter stepped in after German clubs Werder Bremen and Schalke both filed complaints because their Brazilians Diego, 23, and Rafinha, 22, chose to join up with their country's Olympic squad in defiance of the clubs' wishes.

Blatter wrote to all member associations to reiterate the world governing body's stance the release of players aged 23 or under is "mandatory" for the Olympics.

However, the day ended with Werder Bremen insisting there was no legal basis for FIFA's position, while Barcelona also got involved, saying they had the support of the Spanish football league (DFL) over their complaints regarding the selection of Argentina forward Lionel Messi.

But Blatter said in his letter: "The release of players below the age of 23 has always been mandatory for all clubs. The same principle shall apply for Beijing 2008."

The fact this tournament is not included in the co-ordinated international match calendar "does not mean that there is no release obligation for the relevant clubs", he added.

"It would appear to be against the spirit of the Olympic regulations to hinder players under the age of 23, who are actually the core of the squads participating in the Men's Olympic Football Tournament, to take part in the final phase of the event."

However, Werder were unimpressed by Blatter's argument, and the club's director of sport Klaus Allofs said the world governing body would be unable to force clubs to release players.

"As in previous publications from FIFA, Blatter is founding his comments on common law and the particular character of the Olympic Games," Allofs said.

"According to the view of our associations the DFB and the DFL, this release has no (legally) binding nature."

Werder have the support of the German Football League, who have advised them to take the case to the Court of Arbitration for Sport.

"The DFL and DFB are in agreement that the publication by FIFA contains no new content and our view of the rules remains unaltered," said DFL deputy president Holger Hieronymus.

"We recommend that the clubs, if their attempts to deal with the national associations or the appropriate FIFA committee remain unsuccessful or unanswered, to call the CAS directly."

Schalke have also reiterated their opposition to Rafinha playing in Beijing, and the club has written to both the Brazilian Football Association (CBF) and the International Olympic Committee (IOC) to ask the defender not play.

The chorus of voices opposing FIFA grew later on Wednesday when Barcelona released a letter of support they had received from the LFP regarding Messi.

The letter said that clubs were not obliged to release players as the tournament is not on FIFA's official calendar.

The letter continued: "The rules of the Olympic football tournaments, approved in the month of January 2007, by FIFA's Executive Committee for the Games of the Olympiad XXIX, also do not impose any obligation for the release of players."

Messi is with Barcelona on their pre-season tour of Scotland, and Argentinian Football Association (AFA) president Julio Grondona has admitted he can do nothing more than cross his fingers and hope the player travels to China.

The clubs' position has been further strengthened by support from the recently-formed European Club Association (ECA) - the successor to the G-14 as a representative of Europe's 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.