Charleroi are suing world body Fifa after player Abdelmajid Oulmers was injured playing for Morocco in 2004.
A UEFA spokesman said: “It would be the end of the national team game. A World Cup would take place with only Spain, Germany, Italy, France and England.”
A Charleroi victory would mean clubs would become entitled to compensation while their players are away on international duty.
The spokesman, William Gaillard, said international football was “in the end what people like the most”.
“If you ask the English fans if they would like to see one English club win the Champions League or England win a championship, then I think you know the answer,” he said.
Charleroi claim the loss of Oulmers damaged their hopes of winning the Belgian league title in 2004/5 and it wants compensation for having to pay the player’s wages while he was out of action.
In May, the case was moved from a tribunal in Charleroi to the European Court of Justice.
The club are being backed by the G14 group of clubs, who are unhappy with FIFA rules that players must be released for international football without entitlement to financial compensation.
A Charleroi victory “would be the end of national football for any football association outside of the big five in Europe,” Gaillard said.
“I was talking to Brazilian, Scandinavian and Eastern European football experts, and they all said we would never see our stars playing for the national team again.
“Recently the head of the Irish FA said if we have to pay for our stars we would never see Robbie Keane playing for Ireland again.
Arsenal boss Arsene Wenger this week weighed in by likening international coaches to joyriders.
“What the national coaches are doing is like taking the car from his (club manager’s) garage without even asking his permission,” Wenger said.
“They’ll then use his car for 10 days and abandon it in a field without any petrol left in the tank.
“We then have to recover it, but it is broken down. Then, a month later, they’ll come to take your car again — and for good measure you’re expected to be nice about it.”