FIFA president Gianni Infantino said on Tuesday that breakaway Super League clubs cannot be "half in, half out" of the established soccer system, while Real Madrid supremo Florentino Perez insisted he is trying to "save football" with the move.
There have been few voices that have backed the breakaway league, with owners of the 12 teams conspicuous by their absence.
The first senior figure of any club involved to publicly talk about the move was Perez - the new chairman of the Super League - who said football needed to evolve and adapt to the times.
European soccer's governing body UEFA has threatened to ban the 12 clubs. UEFA chief Aleksander Ceferin has not held back on his views of the renegade clubs, who will be guaranteed places in the new competition in contrast to the Champions League which requires teams to qualify via their domestic leagues.
Having labelled the competition a "spit in the face" of football fans, Ceferin insisted, however, that there is still time for reconciliation at the UEFA congress on Tuesday.
At the same congress, the International Olympic Committee warned that the existing structure of European sports is under threat by self-interest and pure commercialism. "It is challenged by a purely profit-driven approach that ignores the... social values of sports and real needs in the post-coronavirus world," IOC President Thomas Bach said.
Real Madrid coach Zinedine Zidane said he will not comment on the proposal while Manchester City manager Pep Guardiola said a European Super League in which 15 clubs cannot be relegated was not sport and has called on those behind the project to explain themselves.
"If you ask me about these teams that have been selected I don't know why. Sport is not a sport when the relation between the effort and the success, the effort and the reward, doesn't exist. So it's not a sport. It's not sport when the success is already guaranteed. It's not sport when it doesn't matter if you lose," said Guardiola.
"Whenever there is a change, there are always people who oppose it... and we are doing this to save football at this critical moment. This is not a league for the rich, it's a league to save football." Real Madrid president , Florentino Perez
Meanwhile, the German Football Association (DFB) on Tuesday demanded the suspension of the 12 clubs until they reconsider.
The Premier League on Tuesday said it has "unanimously and vigorously" rejected plans for a European Super League and is planning to take action against the six English top-flight sides that have signed up for the competition.
Spanish football league chief Javier Tebas has criticised Real Madrid president Florentino Perez for being one of the driving forces behind the European Super League, labelling the proposal as the "death of football".
The Qatari president of French soccer club Paris St Germain - which is not in the 12-team breakaway Super League - said any proposal without UEFA's support would not help football.