The basic ruling is that women are forbidden to use a veil that covers the face in the state of Ihram. However, according to some jurists she is permitted to cover her face using something other than the veil/niqab. ‘Aishah said: “Men on camels used to pass by us while we were with the Prophet (peace and blessings be upon him) and in the state of ihram. We would cover our faces with our gowns when they passed by us, and then uncover them again.”
However, there are many women who are making a visit to the Sacred mosque and so are not in a state of Ihram and thus can wear niqab.