'Firmly against any approval of a revised Club World Cup at this point in time and confirm that no ECA clubs would take part to such a competition.'
Cristiano Ronaldo was recalled by the Portuguese national squad on Friday for the first time since last year's World Cup.
The 34-year-old, who is Portugal's most capped player with 154 appearances and all-time leading scorer with 85 goals, had not been included for any games in the second half of last year.
Coach Fernando Santos had initially said that a mutual decision had been made to give Ronaldo a rest following his move from Real Madrid to Juventus but had been ambiguous about the player's future when questioned in November.
Portugal continued to flourish without Ronaldo, winning their Nations League group against Poland and Italy with a game to spare to qualify for the final four-team tournament which the country will host in June.
However, any doubts about his future were removed when Ronaldo's name was among 25 read by Santos during a news conference for the Euro 2020 qualifiers against Ukraine and Serbia
"Cristiano is the best player in the world and is returning to his team," said Santos. "Any team is stronger with him."
Portugal host Serbia in Lisbon on March 22 and Ukraine three days later.
Barcelona, United, Juventus to boycott FIFA's Club World Cup
Europe's top clubs say they will boycott FIFA's proposed new Club World Cup, leaving FIFA president Gianni Infantino's latest plans in serious doubt and risking a major split in the world game.
European clubs say will boycott FIFA's Club World Cup
Infantino had hoped to get backing this week for creating two new, potentially money-spinning, tournaments -- a new worldwide Nations League for national teams and a fresh 24-team Club World Cup to start with a 'pilot' edition in 2021.
However, even before Friday's meeting of the ruling FIFA Council, Infantino was forced to drop the Nations League plan after his own task force found there is no consensus for such a tournament and a lack of space in the international calendar.
Infantino still wants to move ahead with his plans for a 'pilot edition' of a 24-team Club World Cup, with the council expected to allow the idea to move forward ahead of June's FIFA Congress which would be asked to endorse the plan.
But a letter to UEFA and Infantino from the European Club Association (ECA) head Andrea Agnelli, chairman of Italian side Juventus, and signed by 15 members of the organisation's board, including officials of Manchester United, Barcelona and Ajax Amsterdam, made clear the opposition.
The letter, sections of which were published in the German newspaper Suddeutsche Zeitung says they are "firmly against any approval of a revised Club World Cup at this point in time and confirm that no ECA clubs would take part to such a competition".
The ECA represents 232 European clubs, including all the leading teams who would be expected to feature in a Club World Cup.
The ECA says a decision on potential new competitions can only be made as part of an agreed framework for the international match calendar post-2024.
A spokesperson for the ECA declined to comment on the content of the letter but told Reuters that they would be meeting later this month to discuss the situation.
"We are following developments around the FIFA Council closely. Any decision impacting our members will be discussed and assessed in detail at our upcoming board meeting to be held on 26th March," they said.
The letter also referenced a 2015 agreement between FIFA and the ECA and said that any decision which is against their wishes would "not be in accordance" with it.
The agreement stated that "any changes to the International Match Calendar... will require explicit consent from ECA."
The letter said: "We are therefore certain that FIFA will honour its contractual commitments and we reserve all rights in this regard."
FIFA did not immediately respond to a request for comment. Infantino is expected to hold a news conference following the council meeting on Friday.
The Club World Cup is currently an annual seven-team tournament which attracts minimal interest in Europe -- the centre of the football business.
UEFA, European football's governing body, is unlikely to fight Infantino's plans at Friday's council, with the proposal expected to be passed on to a further meeting ahead of June's congress.
But it is hard to see how a 2021 edition could go ahead without the star attractions -- Europe's top clubs.
The FIFA Council is also expected to decide to continue exploring plans to expand the 2022 World Cup in Qatar to 48 teams with the possibility of additional hosting in other countries.