"AC Milan, Arsenal, Atletico Madrid, Chelsea, Barcelona, Inter Milan, Juventus, Liverpool, Manchester City, Manchester United, Real Madrid and Tottenham Hotspur have all joined as founding clubs," said a statement by the group.
The founding clubs will receive "a one-time payment of the order of 3.5 billion euros".
"Three additional clubs will be invited to join them before the start of the inaugural season which will start as soon as practicable," added the organisers.
UEFA said earlier Sunday that players from the 12 clubs faced international exile and described it as "a cynical project, a project that is founded on the self-interest of a few clubs".
However, the 12 insisted the new competition will benefit the game in general.
If the figure of a 3.5 billion euro windfall is confirmed, it will represent a greater revenue than currently generated by UEFA for all of its club competitions.