The 2026 World Cup will be the first to comprise 48 teams after FIFA unanimously agreed last month to expand the competition from its current 32-team format. A final decision on how many spots each confederation will be given is expected when FIFA meets on March 30, but Mr. Ceferin insisted that Europe’s conditions are clear: UEFA will demand 16 places, an increase of three teams, and request a guarantee that only one European team will be placed in each three-team group.

“Sixteen teams and each European team in a different group is the red line,” he said. “That is what we will insist on. The others are still discussing. That will happen, or we will make things quite complicated.

“Two or three of our member nations suggested to me that we ask for 20 or 24 teams, and if we have to settle for 16, then O.K., but I do not want to push it. I said that we should be realistic. To say that we want half the teams at the World Cup would look arrogant to me.”

Mr. Ceferin also acknowledged that more stringent immigration policies could harm Britain’s chances of hosting major finals once that country exits the European Union. The Champions League final will have been held in Britain three times in the last nine years — twice at Wembley Stadium in London and this June at Millennium Stadium in Cardiff, Wales — and the semifinals and final of the 2020 European Championship are set for London.

“If ‘Brexit’ happens, everything changes,” Mr. Ceferin said. “But football was played before, and it will be played in the future. Now, with free movement in the European Union, it is much better.”

Though Mr. Ceferin, a former president of Slovenia’s soccer federation, insisted that UEFA’s commitment to staging the final matches of Euro 2020 in England remained “firm,” he said the organization might have to discount Britain as a potential venue for showpiece games if “Brexit” made it harder for players and fans to enter the country.

As proof of the extra complications that might arise after England’s exit, he cited the example of Serge Aurier, the Paris St.-Germain defender who was denied entry by British authorities for a Champions League game against Arsenal last year because of a conviction for assault that was under appeal.