There are no official statistics to suggest how widespread the practice of instant divorce is, but the Bharatiya Muslim Mahila Andolan study found that among a sample of more than 4,700 women, 525 were divorced and 404 of those were “triple talaq” divorces. A Muslim woman in India who seeks a divorce must generally gain the permission of her husband, a cleric or other Islamic authorities.

The Quran makes no mention of instant divorce using the talaq method. The practice is outlined in the hadiths, or sayings attributed to the Prophet Muhammad, which are regarded as less authoritative than the Quran but still influential in shaping Islamic doctrine.

Today, instant divorce is not uniformly practiced or accepted in the Muslim world. In many Muslim-majority countries, religious leaders frown on the practice and note that the Quran recommends that couples make a genuine effort to reconcile and resolve their differences before parting ways, said Julie Macfarlane, a law professor at the University of Windsor who wrote a book on Islamic divorce law.

Ms. Khan, 28, whose husband divorced her with his letter to her, has been visiting one Muslim cleric after another, searching for one who will say she and her husband should try to reconcile.

“I am a devout Muslim,” she said, “but I think the Muslim law board is crazy.”

“There’s no way I can ever believe I am divorced by such a letter,” she said as she thumbed limply through photographs of her wedding day. “It cannot be Allah’s will to break up families in this way.”

An instant divorce method available only to men would seem to be obviously discriminatory and an easy ruling for the Supreme Court of India. But it is not so simple.

The All India Muslim Personal Law Board, which presents itself as the protector of Muslim rights, has argued that instant divorce is part of the practice of Islam and thus protected by the Constitution.