They are calling on FIFA·President Sepp Blatter to back their campaign which could put the future of football in the Olympics in jeopardy.
Jacques Rogge, the President of the International Olympic Committee (IOC), has warned Blatter the sport risks elimination if it adopts the controversial proposals to make the tournament an event only open to under-21 players.
He wants to keep the current under-23 format which also allows three over-age players to take part, a format that would Britain to pick someone like Wayne Rooney or, even,·David Beckham at London 2012.
But the·European Club Association (ECA), representing 144 clubs from 53 nations, wants Blatter to stand by statements that the London tournament should be an under-21 event.
Joan Laporta, the President of European champions Barcelona and vice-chairman of the ECA, said:·"We thought it was clearly accepted by FIFA.
"We expect that Mr. Blatter will honour his commitment."
The IOC also wants the Olympic tournament put on FIFA's international calendar which would force clubs to release players.
European clubs object because an Olympic tournament scheduled in July-August interrupts their pre-season planning and qualifying matches for the lucrative Champions League.
Laporta tried to stop its Messi going to China last year to play for Argentina.
Barcelona joined German clubs Schalke and Werder Bremen to defeat FIFA at the Court of Arbitration for Sport, but then gave in to pleas to let the players go.
Laporta said: "Fortunately it was wonderful.
"They [Argentina] won the gold medal and Messi had a marvellous season.
"But I would give you a lot of examples of [clubs] taking risks.
"It's very important to have a clear-cut rule and don't leave the decisions to the clubs.
"Then we get pressure from the players, the national associations and the fans."
June 2009: Rogge wants no change to Olympic football for London 2012
June 2009: Blatter backs down on Olympic age plan for London 2012
September 2008: Europe's top clubs determined to fight FIFA over Olympics
August 2008: Blatter launches review into 2012 Olympic football tournament