12 clubs from England, Spain and Italy have signed up as 'founding members' of the new competition.
Dortmund and Bayern both rejected invitations to join the new league, as did French giants PSG.
"The board members of the European Club Association (ECA) came together for a virtual meeting on Sunday evening, where it was agreed that the board's decision from last Friday still stands," said Watzke.
"This decision dictates that all clubs wish to implement the proposed reforms to the UEFA Champions League. The ECA board members took a clear stance in rejecting plans for the establishment of a Super League."
Watzke went on to confirm Bayern Munich have also opted out of the competition, adding: "Both German clubs on the ECA board, FC Bayern Munich and Borussia Dortmund, shared exactly the same stance throughout all discussions."