Zidane, the France captain, became the fourth player to be sent off in a World Cup final when he rammed his head into the chest of Materazzi knocking him to the ground. A Fifa press release said that there were incidents towards the end of the match that they wanted to clarify.

"Referee Horacio Elizondo sent off French player Zinedine Zidane for an act of violence against Marco Materazzi in the 110th minute," the Fifa statement read. "The incident was directly observed (without the use of a monitor) by fourth official Luis Medina Cantalejo from his position at the pitchside, who informed the referee and his assistants through the communications system. Fifa will open a disciplinary investigation into Zidane's conduct to enable it to clarify the circumstances surrounding the incident as exactly as possible."

The incident took place in the second half of extra-time after Italy broke up a French attack. Zidane and Materazzi were seen to exchange words before Zidane lowered his head to butt Materazzi. Zidane was sent off, reducing France to ten men, and Italy went on to win a penalty shootout 5-3.

Fifa explained that investigations are a regular practice whenever a red card is shown in an international match to determine how the player, and any others who may be involved, should be punished. The governing body declined to comment on whether Materazzi's conduct was also being investigated, or whether it was trying to find out if the Italian player had provoked Zidane.

"I did insult him, it's true," Materazzi said in an Italian newspaper. "But I categorically did not call him a terrorist. I'm not cultured and I don't even know what an Islamic terrorist is."

The 32-year-old Inter Milan player did not elaborate exactly on what he said to Zidane: "It was one of those insults you're told tens of times and that always fly around the pitch."

Meanwhile, Walter Veltroni, the mayor of Rome, accused Fifa of double standards, noting that soccer's governing body named Zidane as the tournament's best player after his head-butt, but that Francesco Totti, the Italy forward, was kicked out of the 2004 European Championship for spitting in an opponent's face.

"I notice a difference in the way in which Totti was treated after the spit and the way in which Zinedine Zidane has been lauded as a champion of soccer, even though he is held responsible for such a violent blow that it could even have had devastating effects," said Veltroni.

Fifa said that the ballot box for the tournament's top player — voted by journalists — was open until after the final had ended, making it impossible to know how many ballots were cast before the match and during it.

Any disciplinary action against Zidane will be largely symbolic as he has already announced his retirement as a professional football player.