An insult about Zinedine Zidane's sister from Marco Materazzi is understood to have provoked the Frenchman's extraordinary headbutt for which he was sent off in Sunday's World Cup final. The Italian was also forced yesterday to deny he called Zidane, whose parents are Algerian immigrants, a " dirty terrorist".

While Zidane is expected to clear up the debate within the next week by revealing what was said to him at the Olympiastadion in Berlin on Sunday the clues are now pointing towards a remark about the Frenchman's sister, Lila. It was a confrontation that began with Materazzi grabbing Zidane's shirt.

It is alleged that Zidane responded by sarcastically telling Materazzi that he could have his shirt as a souvenir at the end of the match. The Italian is alleged to have responded by saying that Zidane could keep it for his sister and then made an extremely derogatory comment about her that version is backed up by lip-readers from the Brazilian TV channel Globo. They claim Materazzi called her a "prostitute".

Materazzi attempted to head off any controversy yesterday by releasing an official statement with the blessing of the Italian football federation after he was accused of making the "dirty terrorist" remark by an anti-racism group in Italy. The SOS Racism group said "very well informed sources from the world of football" had told it that Materazzi had called Zidane a "dirty terrorist".

Materazzi said: "It is absolutely not true, I did not call him a terrorist. I'm ignorant. I don't even know what the word means."