The admission was seized upon by his opponents, who said his years of deception and failure to stand by his wife raised questions about his loyalty and honesty. It has also offered a rare glimpse into his private life, about which little is known.
A retired schoolteacher living in rural Gujarat on a pension of 14,000 rupees a month has claimed in interviews that she is Mr Modi's wife.
The woman, Jashoda Chiman Modi, said she followed Mr Modi's progress on television and in newspapers and still considered herself to be his wife despite being abandoned 42 years ago. She confirmed they had married at 17 - below the legal age of 21 for men - but said they spent just three months together in total during their first three years of marriage.
He initially encouraged her to continue her studies, she said, but then revealed he was leaving and did not want her to join him.
"He told me once that 'I will be travelling across the country and will go as and where I please'. When I came to Vadnagar to live with his family, he told me, `Why did you come to your in-laws' house when you are still so young, you must instead focus on pursuing your studies'," she said.
"When he told me he would be moving around the country as he wished, I told him I would like to join him. However, on many occasions when I went to my in-laws' place, he would not be present and he stopped coming there," she added.
Despite the rejection, they separated without ill-will. "We have never been in touch and we parted on good terms as there were never any fights between us. I will not make up things that are not true," she said.
Mohan Guruswamy, the former BJP manifesto writer, and other analysts said the disclosure had perhaps been made to avoid a likely legal challenge to his nomination. "Half of India will think the worst of him for it... and the other half won't mind. He will deal with it," he said.
Rashid Alvi, the Congress party leader, said the disclosure raised more important questions for a potential Indian leader: "If he can't take care of his wife, how can he take care of the country?"
Mr Modi, whose first job was to help his father sell tea, told a biographer recently that he "actually enjoys loneliness" and has said that because he has no children he is more likely to be a clean politician. "I've no familial ties. Who would I ever try to benefit through corruption?" he told a recent rally.