CAIRO (Reuters) - A research body in Egypts main centre of Sunni learning has backed a ruling on when a Muslim woman should lift her niqab, the veil that virtually hides the whole face, the official MENA news agency reported on Saturday. The issue of when to wear the niqab has generated heated debate in mainly Muslim Egypt, where many women cover their hair with the hijab but where the numbers of those covering their faces too is increasing, even if still not the norm. The furore was sparked this month when Sheikh Mohamed Sayed Tantawi, head of Al-Azhar, the centuries old Sunni centre of learning, told a girl in a classroom to remove the niqab - at first interpreted in some media as an outright ban. Al-Azhar clarified to say it was a ruling against women in all-female classes wearing the most conservative form of veil. On Saturday, Al-Azhars research centre confirmed the ruling and lent its support to the three cases when a woman is banned from wearing the niqab, MENA reported. It said the niqab should be removed when: a girl is in an all female class with women teachers; in exam rooms when all students and supervisors are women; and in all-female dormitories.