ZURICH, Switzerland — FIFA ruled today that professional clubs must allow players 23 or under for the Beijing Olympics, clearing the way for Lionel Messi to play for Argentina despite opposition from FC Barcelona.

Slim Aloulou, a Tunisian judge on FIFA's player status committee, said the release of under 23 players was "mandatory for all clubs."

"Taking part in the Olympic Games is a unique opportunity for all athletes of any sporting discipline," FIFA said in a statement. "It would not be justifiable to prevent any player younger than 23 from participating in such an event if his representative team had qualified."

Apart from the 21-year-old Messi, the decision means that Schalke and Werder Bremen must release Rafinha and Diego, respectively, for Brazil.

The three clubs had challenged the FIFA rules, saying the Olympics were not included on soccer's international match calendar.

But FIFA said the calendar only reflects the schedule for A-teams — those which include players older than 23. The dates for the Beijing tournament are set by the International Olympic Committee, and not by FIFA, it added.

The release of young players has traditionally been accepted by clubs since the rule first came into force 20 years ago.

A FIFA emergency committee separately confirmed the rule Tuesday.

The clubs now have the option of appealing to the Court of Arbitration for Sport.

Prior to Wednesday's ruling, Barcelona had indicated it would go to CAS if the FIFA ruling went against it.

"I am waiting to see what FIFA says and, if it says I don't have to go, I won't go," Messi was quoted as saying on Barcelona's Web site before the ruling. "If it says I have to go, I will go without awaiting the decision of the CAS, because that would be too long for my teammates and squad management to have to wait."

Messi added he would return to Barcelona if CAS then ruled in its favor.

"Barcelona continues with its stance of not wanting to let me go and I understand it," Messi said. "But I also think they have to understand that my dream is to take part in the Olympic Games with the national team."

Barcelona and Schalke both have Champions League qualifiers during the period of the Olympics.

The Olympic tournament will begin one day before the Aug. 8 opening ceremony.