A dozen of the world’s richest and most storied soccer clubs on Sunday announced that they had formed a breakaway European club competition that would, if it comes to fruition, upend the structures, economics and relationships that have bound global soccer for nearly a century.

After months of secret talks, the breakaway teams — which include Real Madrid and Barcelona in Spain; Manchester United and Liverpool in England; and Juventus and A.C. Milan in Italy — confirmed their plans late Sunday. They said they planned to add at least three more founding members, hold midweek matches that would put the league in direct competition with the existing Champions League, and begin play “as soon as practicable.”

“We will help football at every level and take it to its rightful place in the world,” said Real Madrid’s president, Florentino Pérez, who was named the first chairman of what the clubs were calling the Super League.

The league they have agreed to form — an alliance of top clubs closer in concept to closed leagues like the N.F.L. and the N.B.A. than soccer’s current model — would bring about the most significant restructuring of elite European soccer since the 1950s, and could herald the largest transfer of wealth to a small set of teams in modern sports history.