Uefa president Aleksander Ceferin said on Thursday it would be "a bad idea" for Fifa to expand the World Cup to 64 nations for the 2030 tournament.
The 2026 edition in the United States, Mexico and Canada will already see the format increase from 32 teams to 48.
But the head of the Uruguayan football federation, Ignacio Alonso, suggested further expanding the tournament at the last Fifa Council meeting in March.
Football's global governing body said it "had a duty to analyse" the proposal, which European chief Ceferin denounced at Uefa's congress in Belgrade.
"It's maybe even more surprising for me than for you. I think it's a bad idea," Ceferin said.
"I think it's not a good idea for the World Cup itself and it's not a good idea for our qualifiers as well, as you know.
"So, I'm not supporting that idea. I don't know where it came from. It's strange that we didn't know anything before this proposal at the Fifa Council."
The 2030 World Cup will be played in three continents, with Portugal, Spain and Morocco the main hosts.
There will also be three matches in South America – in Argentina, Uruguay and Paraguay – to celebrate the centenary edition. The inaugural World Cup was held in 1930 in Uruguay.
That decision paved the way for Saudi Arabia to hold the 2034 event.