FIFA will open a disciplinary investigation into the circumstances surrounding France [Images] captain Zinedine Zidane's [Images] head-butt on Italy's [Images] Marco Materazzi during the World Cup final.

Playing the last match of a great career, the French midfielder drove his head into Materazzi's chest in the second period of extra time in Berlin after exchanging comments with the Italian.

"FIFA will open a disciplinary investigation into Zidane's conduct to enable it to clarify the circumstances surrounding the incident as exactly as possible," world soccer's governing body said in a statement.

What exactly was said between the two remains a mystery although Materazzi has denied allegations he called Zidane a "dirty terrorist".

"I used an insult that I've heard used a dozen times and which is heard all the time on the pitch," Materazzi told the Gazzetta dello Sport newspaper when asked whether he had insulted Zidane's sister.

"I certainly did not call him a terrorist: I'm ignorant, I don't even know exactly what an Islamic terrorist is ... I also certainly didn't say anything about Zidane's mother: for me, a mother is sacrosanct."

Italy won the final in a penalty shootout after Zidane was sent off for the head-butt.

There was further controversy when the incident was brought to the attention of referee Horacio Elizondo by the fourth official at the match, prompting suggestions that he had used television replays.

FIFA backed the version of the fourth official, who insisted he had seen the foul as it happened. Video technology cannot be used in soccer as a basis for a refereeing decision.

"The incident was directly observed [without the use of a monitor] by fourth official Luis Medina Cantalejo [of Spain] from his position at the pitchside, who informed the referee and his assistants through the communications system," said the world body.

IRONIC TOUCH

In an ironic touch, Zidane was later declared the outstanding player of the tournament, the winner of FIFA's Golden Ball award, in a vote decided by the media.

FIFA could not confirm how many of those votes were cast before the 34-year-old's red card. "The votes were cast into ballot boxes in the media centre in Berlin that remained open until midnight on the day of the final," it said.

"It is impossible to determine how many journalists cast their votes before the match and how many during it, as the ballot papers were not counted until after the deadline for voting had expired."

Zidane has not explained his actions publicly although he is expected to later this week. Materazzi said the incident started when he grabbed Zidane by the shirt.

"He turned round, he said to me with a sneer, looking at me up and down, mockingly 'If you really want my shirt, I'll give it to you afterwards'," the Italian said.