The World Cup will be expanded to host 48 teams, up from 32, Fifa has decided.
An initial stage of 16 groups of three teams will precede a knockout stage for the remaining 32 when the change is made for the 2026 tournament.
The sport’s world governing body voted unanimously in favour of the change at a meeting in Zurich on Tuesday.
The number of tournament matches will rise to 80, from 64, but the eventual winners will still play only seven games.
The tournament will be completed within 32 days – a measure to appease powerful European clubs, who objected to reform because of a crowded international schedule.
The changes mark the first World Cup expansion since 1998.
New Fifa Now, a campaign group that says the governing body needs to reform, labelled the expansion “a money grab and power grab”.
“It will dilute the competitiveness of the tournament and, therefore, the enjoyment of fans,” it said in a statement.
“It will not help development of the game or provide improved competitive opportunities for lower-ranked nations. Instead, it will make a mockery of the qualification process for most confederations.”
Fifa president Gianni Infantino has been behind the move, saying the World Cup has to be “more inclusive”.
Speaking at a sports conference in Dubai in December, Infantino said expansion will also benefit “the development of football all over the world”.
He added: “There is nothing bigger in terms of boosting football in a country than participating in a World Cup.”