A breakaway European Super League would have “serious and lasting implications” for the long-term sustainability of professional football, a group representing more than 900 clubs in 25 countries across Europe has warned.

According to Der Spiegel, several clubs – including Real Madrid and five Premier League teams – have discussed forming a 16-team European Super League that could begin as soon as 2021. As part of their plans, the clubs allegedly discussed “an option for leaving the national leagues and their football associations behind entirely”.

However the Association of European Leagues has condemned the idea, saying it strongly opposed the “creation of any ‘closed and franchised style’ Super League”. In a statement the association also reiterated its support for Uefa, the European football governing body, and the current pyramid structure, in which promotion and relegation and the sporting merits of clubs are at the core of every competition.

“Domestic football is at the heart of the game throughout Europe for all football stakeholders: players, clubs, leagues, national associations and, more importantly, fans,” it added. “Proposals for a closed Super League will have serious and lasting implications for the long-term sustainability of professional football in Europe.

“Within this context, the European Leagues fully supports Uefa in the management and organisation of European club competitions and shares with Uefa the common principle of protecting and enhancing competitive balance in European football.”

Der Spiegel says it has seen a “binding term sheet” sent on 22 October 2018 by the firm Key Capital Partners to Real Madrid’s president, Florentino Pérez, suggesting that the La Liga club would be joined by Barcelona, Manchester United, Manchester City, Chelsea, Arsenal, Liverpool, Bayern Munich, Juventus, Milan and Paris Saint-Germain in the European Super League, and would not face relegation for 20 years.

According to the document a further five clubs – Atlético Madrid, Borussia Dortmund, Marseille, Internazionale and Roma – would also appear as “initial guests”.

On Monday the Uefa president Aleksander Ceferin condemned the plans, saying it would “be boring – and damage football worldwide”.

He told Kicker: “To see Juve versus Bayern every week would be more boring than let’s say Juve versus Torino. It’s no question for me that I will fight and do all I can against such a league for as long as I am here.”