Is it permissible for a woman to perform the obligatory Hajj without a mahram, if one is unavailable?

It is impermissible for a woman to travel without a mahram, even if her journey is for the obligatory Hajj or 'Umrah, due to the Prophet, peace and blessings be upon him, saying: "A woman should not travel except with a mahram." A group of scholars has gone to the extent of saying that the presence of a mahram is a condition for Hajj to be obligatory upon a woman. So, if she does not find a mahram, Hajj is not obligatory upon her, and she is excused.