FIFA is to open a disciplinary investigation into the circumstances surrounding France captain Zinedine Zidane's head-butt on Italy's Marco Materazzi during the World Cup final on Sunday.

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.

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

"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.

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.

In an ironic touch,

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