Uefa has told Croatia that they could be thrown out of Euro 2008 if their supporters direct any racist chants towards England’s players.
Fifa charged the Croatian Football Federation with bringing the game into disrepute after fans formed a human swastika during a recent friendly in Italy.
Uefa spokesman William Gaillard said: “We can impose heavier sanctions if it happens again.
“That ultimately could be exclusion from the competition.”
England’s black players have been subjected to racist abuse in the past, most infamously when Ashley Cole was targeted by Spain fans in a friendly game in Madrid.
Uefa has contacted the Croatian government to reinforce their warning, while their football federation has also implemented measures to make sure there are no problems in Wednesday’s game against England.
Croatia supporters have a history of making racist gestures and the Croatian Football Federation was fined during Euro 2004 over racist banners displayed at a game against France.
“There are new rules, which can lead to exclusion from a competition and docking points in extreme cases,” Gaillard added. “We will monitor events closely.
“Ourselves and Fifa have imposed sanctions and the sanctions will be heavier in the future.”