UEFA have started a disciplinary process against Spain captain Álvaro Morata and player of the tournament Rodri for singing about a sovereignty claim on Gibraltar at the team's European Championship title celebration.
UEFA said a disciplinary inspector "is being appointed to evaluate a potential violation" of its rules by the two players.
Morata and Rodri led thousands of fans singing "Gibraltar is Spanish" at a celebration in Madrid on Monday after the team's 2-1 win over England in the Euro 2024 final.
Gibraltar on the southern tip of Spain has been a British overseas territory for more than 300 years.
The Gibraltar football federation filed a formal complaint to UEFA on Thursday.
UEFA gave no timetable for a possible case which could see the players suspended from Nations League games in September.
At Euro 2024 in Germany, UEFA banned Albania player Mirlind Daku for two games after leading fans in nationalist chants against Serbia and North Macedonia.
Morata completed a move from Atletico Madrid to AC Milan on Friday.