UEFA president Aleksander Ceferin has again underlined his opposition to World Cups every two years, saying the tournament's rarity is its most attractive quality.
FIFA is holding a feasibility study into shortening the gap between men's and women's World Cups from four to two years, following a request from the Saudi Arabian federation in May which was approved by 166 national associations.
Ceferin admitted in a letter to a fans' group last week to having "grave concerns" at the plans, which have been endorsed by FIFA's head of global development Arsene Wenger.
- UEFA to oppose FIFA plans for World Cup every two years
- Brazil-Argentina suspended after health officials enter pitch
The UEFA chief said at the opening of the European Club Association (ECA) General Assembly on Monday: "We think that the jewel of the World Cup has value precisely because of its rarity.
"Holding it every two years will lead to less legitimacy and dilute the World Cup itself. We think there is a space for everything and both national teams and clubs are fully occupying that physical and commercial space."
Ceferin also highlighted his concerns over the impact on players, adding: "They don't need to see more of their summers spent on consuming tournaments rather than devoted to relaxing and recuperation."
ECA chairman Nasser Al Khelaifi also called for "honest engagement" on the international match calendar and not "unilateral decisions".
FIFA president Gianni Infantino insisted the ECA clubs, and all of football's stakeholders, would be consulted but insisted there was overwhelming evidence for a need to alter the calendar.
"There are no taboo topics, the door of FIFA is open to any idea to any proposal," he said in a recorded address to the assembly.
"We shouldn't take this consultation process as any sort of challenge, any sort of fight, as I've been hearing here and there.
"It is simply a way to try to make football, and global football, strong, and for this we need the help and the assistance of everyone.
"We are, and we will be, approaching you and we hope we can find something which suits, of course, the European clubs but also the clubs from all over the world, the associations from all over the world."
Infantino said that the events preceding and during the September international break highlighted the need for flexibility, citing the row over player release from European clubs to South American countries in particular, the "crazy" suspension of the Brazil versus Argentina World Cup qualifier due to a quarantine issue and the coup in Guinea which forced that country's match against Morocco to be postponed.
"We have to try to find compromises," he said.
"This September window has not been ideal, by far not - let's try to find a good compromise for October for November.
"To be FIFA president, to be in FIFA, requires some flexibility, some adaptability and you have to face some challenges for which you were not prepared and I want to ask you, humbly, to consider this as well."
Al Khelaifi: European clubs to have access to debt fund
Hard-hit European teams are set to have access to a debt fund worth billions of euros, according to the chairman of the European Club Association, Nasser Al Khelaifi.
The continent's clubs are trying to recover from two seasons starved of various income streams due to the coronavirus pandemic, and Paris Saint-Germain president Al Khelaifi announced UEFA and the ECA is working on a plan to help clubs secure competitive finance to see them through in the short term.
Al Khelaifi said the fund would "allow clubs of all tiers to accelerate their recovery from the financial devastation of Covid".
"We know the need from our members is significant, and we have fought strongly for this project," he said.
"I would like all of your support so we can quickly put the closing elements in place with UEFA. We can then provide simple and fast support to help overcome the liquidity crisis that is still engulfing our clubs and the communities they support."
Al Khelaifi and UEFA president Ceferin hinted at how new financial regulation for the medium and long term could look too in their addresses to the ECA's General Assembly.
The Times reported in August that UEFA was considering a wage cap set at 70 per cent of revenue, with clubs able to pay a "luxury tax" on any spending above the cap, which would have to be adhered to by any club wishing to enter European competition.
It would represent a major shift away from the current 'break-even' model of financial fair play, which has been criticised as a means by which the traditional giants of European football are able to maintain their status.
Ceferin said: "It is time to question the old ways and the traditional measures. It is now time to seriously work together to put in place a true direct cost control system.
"I'm of the view that the cost control system of checks and balances, limits and allowances can be responsible, accountable and, importantly, transparent."
The financial model for European football will also be discussed at the UEFA Convention later this week.
Ceferin and Al Khelaifi's speeches touched on the Super League scandal from April. Ceferin described those behind the league as "disgraceful and charlatan".
In a reference to Al Khelaifi's predecessor as ECA chairman Andrea Agnelli, one of the key movers behind the Super League and a former ally, Ceferin said: "When passing through a storm. You need a good captain and the ECA has a good captain. The chairman that took the wheel when the ex-captain escaped from the ship."
Al Khelaifi said he did not want to focus on the "fabulists and failures" behind the Super League for too long.
"While the three rebel clubs waste energies, twist narratives and continue to shout at the sky, the rest of us are moving forward and focusing every energy onbuilding a better future for European football - together as one," he said.
"As you know, for the nine clubs who asked to come back into our family, the ECA board has reintegrated them into our structures with renewed commitments to strengthen our association. I welcome them back to the ECA family."