Twelve teams have so far confirmed they will be part of a new European Super League competition.
Who is involved?
The "founding clubs" are AC Milan, Arsenal, Atlético de Madrid, Chelsea, FC Barcelona, FC Internazionale Milano, Juventus, Liverpool, Manchester City, Manchester United, Real Madrid and Tottenham Hotspur.
Three more clubs are expected to join before the first season – and another five will qualify annually based on their performance during the previous season.
No German or French clubs have signed up so far.
What is the format for the competition?
The clubs will play midweek fixtures while continuing to play in their national leagues.
Starting in August, two groups of 10 clubs will play home and away fixtures, with the top three in each group qualifying for the quarter-finals.
The teams finishing fourth and fifth will compete in a two-legged play-off for the remaining quarter-final positions.
A two-leg knockout format will be used to reach the final at the end of May, which would be a single fixture at a neutral venue.
What about women’s football?
A women’s tournament will be launched "as soon as practicable after the start of the men’s competition" to "advance and develop the women’s game", the league said.
The Super League said the coronavirus pandemic has "accelerated the instability in the existing European football economic model", also showing that a "sustainable commercial approach" is needed to support the "European football pyramid".
It said: "In recent months extensive dialogue has taken place with football stakeholders regarding the future format of European competitions.
"The founding clubs believe the solutions proposed following these talks do not solve fundamental issues, including the need to provide higher quality matches and additional financial resources for the overall football pyramid."
What’s in it for the clubs?
The league’s statement on Sunday night said the new tournament would provide "significantly greater economic growth and support" for European football.
This would include uncapped "solidarity payments", which would be "substantially higher" than those in the current European competition and would grow in line with league revenues.
The exact amount was not specified but the league said it was "expected to be in excess of €10bn (£8.6bn)" during the "initial commitment period".
The league also promised a further €3.5bn (£3bn) for founding clubs to "support their infrastructure investment plans and to offset the impact of the COVID pandemic".
Many football clubs have faced significant revenue losses while live sport was suspended or limited because of the pandemic.
The Super League hopes to generate €4bn (£3.5bn) annually from broadcasters, with the founding clubs taking the greatest slice.
This compares to UEFA’s total commercial revenue of €3.25bn (£2.8bn) for each of the past three seasons from selling the rights to the Champions League, Europa League and UEFA Super Cup.
What have football bodies said?
On Sunday night, FIFA said: "Any football competition, whether national, regional or global, should always reflect the core principles of solidarity, inclusivity, integrity and equitable financial redistribution.
"Moreover, the governing bodies of football should employ all lawful, sporting and diplomatic means to ensure this remains the case."
The English FA said the move would be "damaging to English and European football at all levels" and that it would "attack the principles of open competition and sporting merit which are fundamental to competitive sport".
It added: "We would not provide permission to any competition that would be damaging to English football, and will take any legal and/or regulatory action necessary to protect the broader interests of the game."
UEFA said: "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.
"As previously announced by FIFA and the six federations, the clubs concerned will be banned from playing in any other competition at domestic, European or world level, and their players could be denied the opportunity to represent their national teams.
"We thank those clubs in other countries, especially the French and German clubs, who have refused to sign up to this. We call on all lovers of football, supporters and politicians, to join us in fighting against such a project if it were to be announced. This persistent self-interest of a few has been going on for too long. Enough is enough."
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."
So why the criticism?
Prime Minister Boris Johnson said the new league would "strike at the heart of the domestic game, and will concern fans across the country" – but why?
Critics have said the league is a power grab by the top clubs and that the closed competition is a way for those clubs to ensure more money for themselves.
Fans are upset that the competition will lose the thrill of promotion and relegation seen in other football leagues and that, consequently, the game will lose its excitement.
With only a few spots available each year for teams outside the main group, it is feared the competition will be closed to most, leaving an exclusive, elite and untouchable group of clubs at the top.
Labour leader and Arsenal fan Sir Keir Starmer said: "This proposal risks shutting the door on fans for good, reducing them to mere spectators and consumers."
Sir Ed Davey, leader of the Liberal Democrats, said: "This is greed personified, ripping the heart out of the English game, leaving clubs up and down the country to suffer after an awful year."