A dozen of the world’s richest and most storied soccer clubs on Sunday announced that they had formed a breakaway European club competition that would, if it comes to fruition, upend the structures, economics and relationships that have bound global soccer for nearly a century.

After months of secret talks, the breakaway teams — which include Real Madrid and Barcelona in Spain; Manchester United and Liverpool in England; and Juventus and A.C. Milan in Italy — confirmed their plans late Sunday. They said they planned to add at least three more founding members, hold midweek matches that would put the league in direct competition with the existing Champions League, and begin play “as soon as practicable.”

“We will help football at every level and take it to its rightful place in the world,” said Real Madrid’s president, Florentino Pérez, who was named the first chairman of what the clubs were calling the Super League, to New York Times.

The league they have agreed to form — an alliance of top clubs closer in concept to closed leagues like the N.F.L. and the N.B.A. than soccer’s current model — would bring about the most significant restructuring of elite European soccer since the 1950s, and could herald the largest transfer of wealth to a small set of teams in modern sports history reported New York Times.

In its current form, European soccer supplements domestic league play — an English league for English teams, a Spanish one for Spanish clubs — with Continental competitions between the best clubs. The most prestigious of those, the Champions League, brings together the best teams from each domestic league each year to play for the title of Europe’s, and arguably the world’s, best club.

The current system funnels hundreds of millions of dollars of annual television and sponsorship revenue to the world’s richest clubs, which supplement their domestic revenue with multimillion-dollar payouts from the Champions League. But the format also sustains smaller teams in each country, which benefit from the gloss of their encounters with the giants and share in the money those teams bring in from broadcasters.

The new superleague model would change that, by stripping the Champions League of its most attractive and most successful teams and effectively walling off the richest clubs in their own closed competition — and allowing them to split the billions of dollars in annual revenue among themselves. According to the Super League’s announcement, the founding clubs will split 3.5 billion euros (almost $4.2 billion) for signing on to establish “a sustainable financial foundation.” The per-team figure means each founding club will receive about $400 million — more than four times what the Champions League winner took home in 2020.

The 12 teams that signed up as founders are, for the moment, limited to a dozen clubs from Spain, Italy and England. A cohort of six teams from the Premier League — United, Liverpool, Manchester City, Arsenal, Chelsea and Tottenham — represents the biggest grouping from a single country. Atlético Madrid is the other team from Spain that is said to have endorsed the project, while the Milan rivals Internazionale and A.C. Milan would join Juventus as Italy’s representatives.

In its current form, European soccer supplements domestic league play — an English league for English teams, a Spanish one for Spanish clubs — with Continental competitions between the best clubs. The most prestigious of those, the Champions League, brings together the best teams from each domestic league each year to play for the title of Europe’s, and arguably the world’s, best club.

The current system funnels hundreds of millions of dollars of annual television and sponsorship revenue to the world’s richest clubs, which supplement their domestic revenue with multimillion-dollar payouts from the Champions League. But the format also sustains smaller teams in each country, which benefit from the gloss of their encounters with the giants and share in the money those teams bring in from broadcasters.

The new superleague model would change that, by stripping the Champions League of its most attractive and most successful teams and effectively walling off the richest clubs in their own closed competition — and allowing them to split the billions of dollars in annual revenue among themselves. According to the Super League’s announcement, the founding clubs will split 3.5 billion euros (almost $4.2 billion) for signing on to establish “a sustainable financial foundation.” The per-team figure means each founding club will receive about $400 million — more than four times what the Champions League winner took home in 2020.

The 12 teams that signed up as founders are, for the moment, limited to a dozen clubs from Spain, Italy and England. A cohort of six teams from the Premier League — United, Liverpool, Manchester City, Arsenal, Chelsea and Tottenham — represents the biggest grouping from a single country. Atlético Madrid is the other team from Spain that is said to have endorsed the project, while the Milan rivals Internazionale and A.C. Milan would join Juventus as Italy’s representatives.

Three more clubs will join as founding — and thus permanent — members, organizers said, and a qualifying mechanism will be created to fill the five other places in the 20-team Super League each season.

A women’s league will be started as well, the announcement said, presumably including the women’s sides of many of the same clubs.

European soccer officials moved quickly to try to block the project. The Premier League condemned the concept in a statement on Sunday and also sent a letter to its 20 member clubs warning them not to take part. Officials at European soccer’s governing body, UEFA, which runs the Champions League, labeled the proposal for a closed superleague a “cynical project” in a statement.

The missive was co-signed by the Premier League, La Liga in Spain and Italy’s Serie A, as well as the soccer federations of each country. Within hours, France’s federation and the French league had added their voices to the growing opposition inside key European soccer circles. Politicians, including Britain’s prime minister, Boris Johnson, and the French president, Emmanuel Macron, also weighed in to denounce the plans, New York Times reported.

But UEFA was taking the threat seriously. Its leaders spent the weekend in discussions about how to block the plan, including banning the breakaway teams from their domestic leagues and blocking their players from competing for their national teams in events like the World Cup. European officials also pointedly reminded the prospective superleague clubs (and, effectively, their players) that soccer’s global governing body, FIFA, has backed their threats of expulsion.

FIFA on Sunday expressed its “disapproval” of the concept of a closed league, but refrained from the type of threats being lobbed by top officials in Europe.

“We will consider all measures available to us, at all levels, both judicial and sporting in order to prevent this happening,” the UEFA statement said. “Football is based on open competitions and sporting merit; it cannot be any other way.”

At the same time, soccer officials also began contacting lawmakers at the European Union, hoping the bloc would be able to strengthen its hand in preserving the status quo.

The leaders of the breakaway group have been trying to get other top teams, like Germany’s Bayern Munich and Borussia Dortmund and the French champion Paris St.-Germain, to commit. But to date those clubs — and others — have declined to walk away from the domestic structures and continental competitions that have underpinned European soccer for generations.

Their concerns can be political and financial. P.S.G.’s president, Nasser al-Khelaifi, sits on the UEFA board, for example, and also heads beIN Media Group, the Qatar-based television network that has paid millions of dollars for the broadcast rights to games in the Champions League and various domestic competitions.

The Premier League wrote to its 20 clubs after its board meeting on Sunday, though, warning the teams that the league’s rules bar clubs from joining outside competitions without approval.

The repercussions of a split between European soccer and its best-known, best-followed and most deep-pocketed clubs would be seismic. Without the top teams, UEFA and the domestic leagues would face demands for millions of dollars in refunds from the broadcasters who pay billions for television rights to air their tournaments. The clubs left out would face a serious blow to their budgets while many are still wrestling with the financial wreckage caused by the coronavirus pandemic. And any ban on national team play would affect players individually, even if they had no role in the decision-making.

Among the most notable teams involved in the breakaway group is Juventus, the serial Italian champion. Its chairman, Andrea Agnelli, had been until Sunday — when he resigned from both posts — a member of UEFA’s executive board and also the head of the European Club Association, an umbrella body for more than 200 top division clubs, the majority of which will be left out of the proposed Super League.

When asked by The New York Times this year to discuss his role in the talks of a breakaway league, Agnelli brushed off the idea as a “rumor.”

Still, according to documents reviewed by The Times in January, plans for the breakaway league had gathered pace since last summer. Top clubs sought to take advantage of uncertainty in the soccer industry caused by the pandemic to forge a new path that would ensure a degree of financial stability for them but would also almost certainly lead to a significant — and potentially devastating — loss in value and revenue for the teams excluded from the project.

Each of the would-be permanent members of the proposed superleague was promised 350 million euros, or $425 million, to sign up, the documents said. The group leading the effort had entered into discussions with JPMorgan Chase to raise financing for the project, according to people with knowledge of the matter. The firm has so far declined to comment.

Earlier this year, UEFA found a powerful ally in opposition to the plans in FIFA, which warned that any player who took part in such an unsanctioned league would be barred from appearing in any of its tournaments, including the World Cup. The statement came after UEFA’s president, Aleksander Ceferin, demanded support from his FIFA counterpart, Gianni Infantino, amid mounting speculation that the breakaway group had FIFA’s support.

By Sunday night, even the alliance of clubs led by Juventus’s Agnelli, the E.C.A., appeared to reject his idea.

Significant hurdles to the plan’s implementation remain. Governing bodies and leagues could follow through on their threats to expel the clubs and their players. As member-owned clubs, Barcelona and Real Madrid would most likely require the support of the thousands of their supporters before formally joining, and any German clubs that agree to take part would face similar obstacles. All can expect heavy internal opposition, too; fan groups across Europe have consistently opposed even the idea of a closed superleague.

On Sunday, one umbrella fan group, Football Supporters Europe, called the superleague idea “illegitimate, irresponsible, and anti-competitive by design.”

“More to the point, it is driven exclusively by greed,” the group said. “The only ones who stand to gain are hedge funds, oligarchs and a handful of already wealthy clubs, many of which perform poorly in their own domestic leagues despite their inbuilt advantage.”