Real Madrid have strongly rejected plans to play Villarreal's La Liga match with Barcelona in the United States, saying it "sets an unacceptable precedent".
The fixture is scheduled for 21 December at Villarreal's Estadio de la Ceramica, but the Spanish football federation (RFEF) approved the proposal on Monday to host the game in America.
The RFEF will now seek permission from Fifa and Uefa to switch it to the Hard Rock Stadium in Miami, which is set to host seven matches at the 2026 World Cup.
But Real Madrid have expressed their "strongest rejection of the proposal" to play the match outside Spain.
In a statement, the club, external said: "If this proposal is carried out, its consequences would be so serious that they would mean a before and after for the world of football."
The Spanish football supporters' association (FASFE) has also joined Barcelona and Villarreal fan groups to make clear their firm opposition to the proposal, warning they would "take appropriate legal action" if it went ahead.
However, Villarreal president Fernando Roig told the club's website the US opportunity is something they "must seize".
"We have to expand the brand of our football and of Villarreal CF," he added.
"Many clubs agree on the need to internationalise and to look for ways to generate more revenue so we can get closer to other leagues, such as the Premier League."
Fifa's rules currently do not allow domestic league matches to be played abroad, but last year it set up a working group to explore the matter
