UEFA's 55 member associations have voted to boycott World Cups if FIFA proceeds with its plan to sell stakes in its competitions to private investors.
The decision was made at an emergency meeting called to discuss the proposals announced by FIFA - world football's governing body - on Tuesday.
UEFA, which governs European football, had made its opposition clear by releasing two damning statements about the plans - and that strength of feeling has now been reaffirmed.
The boycott would cover all FIFA competitions, including the men's and women's World Cups and Club World Cup, and be triggered if FIFA president Gianni Infantino's proposals are voted through by member associations.
The first time this stance will be tested is October, when the Women's World Cup play-offs are due to be held.
In a statement released after the meeting, UEFA said it and its 55 member associations "stand as one".
"We unanimously and unequivocally reject FIFA's proposal to transfer ownership interests in the World Cup and other FIFA competitions to private investors," it said.
"The World Cup cannot be treated as an investment product. It is one of football's greatest sporting legacies. It has been built over generations by players, national teams and supporters across every continent. No part of it should ever be surrendered to private investors. The World Cup is not for sale."
FIFA wants to create a commercial subsidiary to run its main events, including the World Cup, and external investors will be able to buy stakes in it.
Infantino wrote to all 211 FIFA member associations saying they would receive $40m (£30m) if they backed his controversial plan to sell off stakes in its major competitions.
The Swiss set a deadline of 19 September for football federations to accept his plans if they wanted to access an initial $20m (£15m).
In response on Wednesday, UEFA accused FIFA of using football "to enrich themselves and their friends".
