(CNN) Football fans, politicians and governing bodies are united in fury after 12 of the sport's biggest teams announced plans to breakaway from European football competitions and form their own "Super League" -- a move that poses an existential threat to the world's favorite sport.
Here's a rundown of what you need to know about the plans, and why they matter.
It is potentially the biggest ever shake up of European club football.
On Sunday, 12 of the largest -- and wealthiest -- football clubs in the world announced their intention to launch a European Super League (ESL).
The league would feature 20 clubs, which would include 15 founding teams, who would be permanent members. A further five clubs would qualify annually based on their achievements in the previous season.
It's unclear whether those five clubs would be relegated from the ESL regardless of performance.
The clubs involved include the traditional "Big Six" of the Premier League: Arsenal, Chelsea, Manchester City, Manchester United, Liverpool and Tottenham Hotspur.
Spanish giants Real Madrid, Barcelona and Atlético Madrid would also feature, alongside Italian sides AC Milan and Inter Milan, as well as Juventus.
The 2020 Champions League winner Bayern Munich is not part of the project, while Borussia Dortmund chairman Hans-Joachim Watzke confirmed that the club was against the formation of the ESL.
Both Bayern and Dortmund are represented on the European Club Association board and, according to Watzke, the two Bundesliga teams support a new Champions League format, which was scheduled to be approved by UEFA on Monday.
Paris Saint-Germain are also not included, with The Guardian reporting their reluctance "could be down to proposed stringent financial regulations in the new competition that would be similar to UEFA's financial fair play."
The timing may be key to understanding the announcement, as UEFA was set to release details of the expanded Champions League format today, outlining an increased amount of football matches in an already congested fixture list.
Football writers and analysts have speculated that the curiously timed declaration of plans for the ESL could be a tactical opening salvo in negotiations with UEFA, used as a bargaining tool to discuss a larger piece of the pie for the top European clubs.
From a fan's perspective, the timing is awful and tantamount to owners exploiting a system weakened by the pandemic. Both grassroots football and the wider pyramids of the game across European national leagues are hemorrhaging money and under threat without fans in attendance for over a year.
Owners of the world's biggest clubs have long agitated for a bigger share of football's TV revenues and other financial rewards, while the increasingly undeniable importance of money in the game has grated with more traditional supporters.
In recent decades, multi-billion dollar takeovers of several teams like Manchester City and Chelsea have widened the gap between football's haves and have nots, and it's extremely rare for a team outside the small group of elites to win a major league trophy.
That disparity has led to rumors of a "super league" for years, and some have suggested the clubs involved could be convinced to shelve the plans in favor of a financial compromise. But Sunday's announcement is by far the closest football has ever come to such a drastic breakaway.
READ: Jose Mourinho sacked as Tottenham Hotspur manager
JP Morgan confirmed to CNN on Monday that it will be financing the proposed new breakaway European Super League, but declined to offer further comment regarding the nature of the deal.
The official statement from the 12 founding clubs said: "Founding clubs will receive an amount of €3.5B ($4.21B) solely to support their infrastructure investment plans and to offset the impact of the Covid pandemic."
"The new annual tournament will provide significantly greater economic growth and support for European football ... and [solidarity payments] are expected to be in excess of €10B ($12B) during the course of the initial commitment period of the clubs."
In the statement, the current clubs involved outlined plans to potentially start the competition in August 2021.
All clubs in the new ESL format would compete against each other midweek, in both home and away fixtures, similar to the current Champions League knockout stage format.
Details released so far have not included a broadcaster set to support the breakaway competition, but the seismic development bears similarities to the establishment of the Premier League -- which was key to BSkyB exerting themselves as the dominant domestic UK broadcaster of football.
With the growth of powerful tech giants and popular streaming services such as Apple TV+, Netflix, and Amazon's Prime Video, the new ESL competition could provide a similar opportunity for such platforms to enter the European sporting market, while also increasing the huge financial broadcast deals that have continued to grow over the past decades, forming the basis for the game's wealth.
READ: The breakaway competition throwing elite game into turmoil
The condemnation of proposals has been widespread.
A statement from FIFA said, "FIFA can only express its disapproval to a 'closed European breakaway league' outside of the international football structures and not respecting the aforementioned principles."
A previous statement from FIFA in January had gone so far as to say that "any club or player involved in such a competition would as a consequence not be allowed to participate in any competition organized by FIFA or their respective confederation."
That would mean that many of the world's best players couldn't play for their country -- and would leave the next World Cup without stars like Cristiano Ronaldo, Lionel Messi, Kevin De Bruyne and many more.
UEFA -- which oversees all European football -- along with the English, Spanish, and Italian governing bodies and the top flight leagues from those three countries co-signed a statement.
"We wish to reiterate that we -- UEFA, the English FA, RFEF, FIGC, the Premier League, LaLiga, Lega Serie A, but also FIFA and all our member associations -- will remain united in our efforts to stop this cynical project, a project that is founded on the self-interest of a few clubs at a time when society needs solidarity more than ever," the statement reads in part.
"We will consider all measures available to us, at all levels, both judicial and sporting in order to prevent this happening. Football is based on open competitions and sporting merit; it cannot be any other way."
The Premier League said, "Fans of any club in England and across Europe can currently dream that their team may climb to the top and play against the best. We believe that the concept of a European Super League would destroy this dream."
England's Football Association said, "It is clear that this would be damaging to English and European football at all levels and will attack the principles of open competition and sporting merit which are fundamental to competitive sport."
On the plans for ESL, UK Prime Minister Boris Johnson said, "They would strike at the heart of the domestic game, and will concern fans across the country. The clubs involved must answer to their fans and the wider footballing community before taking any further steps."
French President Emmanuel Macron said in a statement, according to Reuters, "The president of the Republic welcomes the position of French clubs to refuse to participate to a European football Super League project that threatens the principle of solidarity and sporting merit."