Arsenal, Chelsea, Liverpool, Manchester City, Manchester United and Tottenham are among 12 clubs who have agreed to join a new European Super League (ESL).
In a seismic move for European football, the Premier League clubs will join AC Milan, Atletico Madrid, Barcelona, Inter Milan, Juventus and Real Madrid.
The ESL said the founding clubs had agreed to establish a "new midweek competition" with teams continuing to "compete in their respective national leagues".
It said the inaugural season was "intended to commence as soon as practicable" and "anticipated that a further three clubs" would join the breakaway.
The ESL said it also planned to launch a women's competition as soon as possible after the men's tournament starts.
UK Prime Minister Boris Johnson, Uefa and the Premier League condemned the move when the news broke on Sunday.
World governing body Fifa had previously said it would not recognise such a competition, and any players involved could be denied the chance to play at a World Cup.
Uefa, Europe's governing body, reiterated that warning on Sunday when it said players involved would be banned from all other competitions at domestic, European or world level and could be prevented from representing their national teams.
After the Super League was announced, Fifa expressed its "disapproval" of the proposed competition and called on "all parties involved in heated discussions to engage in calm, constructive and balanced dialogue for the good of the game".
In a statement, the ESL said: "Going forward, the founding clubs look forward to holding discussions with Uefa and Fifa to work together in partnership to deliver the best outcomes for the new league and for football as a whole."
There were talks in October, involving Wall Street bank JP Morgan, over a new Â£4.6bn competition that would replace the Champions League.
Uefa had hoped the plans for a new 36-team Champions League - with reforms set to be confirmed on Monday - would head off the formation of a Super League.
However, the 12 sides involved in the Super League do not think the reforms go far enough.
They said the global pandemic had "accelerated the instability in the existing European football economic model".
"In recent months, extensive dialogue has taken place with football stakeholders regarding the future format of European competitions," they added.
"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 is the proposed format?
The league will have 20 teams - the 12 founding members plus the three unnamed clubs they expect to join soon, and five sides who qualify annually according to their domestic achievements.
Under the proposals, the ESL campaign would start in August each year, with midweek fixtures, and the clubs would be split into two groups of 10, playing each other home and away.
The top three in each group would qualify for the quarter-finals, with the teams in fourth and fifth playing a two-legged play-off for the two remaining spots.
From then on, it would have the same two-leg knockout format used in the Champions League before a single-leg final in May at a neutral venue.
The ESL said it would generate more money than the Champions League and would result in a greater distribution of revenue throughout the game.
What do the Super League leaders say?
Real Madrid president Florentino Perez, the first chairman of the ESL, said the new competition would "help football at every level".
"Football is the only global sport in the world with more than four billion fans and our responsibility as big clubs is to respond to their desires," he added.
Juventus chairman Andrea Agnelli has resigned from the Uefa executive committee and as chairman of the European Club Association (ECA), which had pushed the planned Champions League reforms.
He said the 12 clubs had "come together at this critical moment, enabling European competition to be transformed, putting the game we love on a sustainable footing for the long-term future".
Manchester United co-chairman Joel Glazer will be a vice-chairman of the Super League.
He said: "By bringing together the world's greatest clubs and players to play each other throughout the season, the Super League will open a new chapter for European football, ensuring world-class competition and facilities, and increased financial support for the wider football pyramid."
What has been the reaction?
Essentially, widespread condemnation from anyone not involved in the proposed league.
Johnson said the plans would be "very damaging for football" and France's President Emmanuel Macron welcomed French clubs refusing to join.
Uefa released a joint statement with England's Football Association, the Premier League, the Spanish Football Federation, La Liga and the Italian Football Federation, as well as Serie A, saying they would "remain united" in trying to stop the breakaway, using "all available measures".
The ECA said it "strongly opposed" the league, while the Football Supporters' Association said the plans were "motivated by nothing but cynical greed".
Among ex-professionals, former Liverpool and Tottenham midfielder Danny Murphy told BBC Sport the plans "sound soulless", former Manchester United captain Gary Neville told Sky Sports he was "absolutely disgusted", while former team-mate Rio Ferdinand said on BT Sport that the proposals will hurt fans the most.
'The ultimate betrayal'
Fan groups associated with all six English clubs involved are strongly opposed to the Super League.
Liverpool supporters' group Spirit of Shankly (SOS) said it was "appalled" by the decision of Fenway Sports Group, the club's US-based owner.
In a social media post, SOS said: "FSG have ignored fans in their relentless and greedy pursuit of money. Football is ours, not theirs. Our football club is ours not theirs."
Chelsea Supporters' Trust called the move "unforgivable" and said its members and "football supporters across the world have experienced the ultimate betrayal".
The Arsenal Supporters' Trust called the club's agreement to join "the death of Arsenal as a sporting institution".
Manchester City's Official Supporters Club said the move showed "those involved have zero regard for the game's traditions", adding it was "determined to fight against this proposed Super League".
The Manchester United Supporters' Trust had earlier said the proposals were "completely unacceptable" and the ESL "goes against everything football, and Manchester United, should stand for".
Tottenham Hotspur Supporters' Trust said the ESL was a "concept driven by avarice and self-interest at the expense of the intrinsic values of the game we hold so dear".