The EU's top court has heard arguments against UEFA from the organisers of the short-lived breakaway Super League.
Lawyers for the Super League have accused Europe's football governing body of being a "monopolistic entity" and abusing its market dominance.
UEFA's has retorted in court in Luxembourg, comparing the clubs behind the Super League to a "cartel" and arguing that it has defended grassroots interests in the sport.
"A league closed to the richest clubs is incompatible with the European sporting model, which is based on merit," lawyer Donald Slater said on Monday.
The trial could result in the biggest upheaval in European football for more than 25 years.
The Super League project failed at launch in April 2021 after most of the 12 "rebellious" clubs reversed decisions on their membership.
But the company formed by the clubs -- now led by Real Madrid, Barcelona and Juventus -- have renewed their legal arguments at the Court of Justice of the European Union.
A verdict in the trial is not expected for several months and will not affect the 12 Super League clubs, who are all playing in UEFA competitions next season.
The dispute between UEFA, world football body FIFA and the Super League will also be closely watched by other sports amid increasing global revenues.