The admission was seized upon by Modi's 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 ($270) a month has claimed in interviews that she is Modi's wife.
The woman, Jashoda Chiman Modi, said she followed Modi's progress on television and in newspapers and still considered herself to be his wife despite being abandoned 42 years ago.
Voters went to the polls on Thursday on the first major day of India's marathon national election. It was the third of nine phases of voting in the world's biggest election that will end when results are published on May 16. Ballots were cast in the capital, New Delhi, with its 17 million residents, and in densely populated rural constituencies in the north.
Early voting took place peacefully and slowly in New Delhi, where a fifth of Parliament's 543 seats are up for grabs, after authorities declared a public holiday, with voters making their way to polling stations in bright spring sunshine.
The race in the capital is being closely watched as it is seen as a three-way contest between the ruling Congress Party, the opposition Bharatiya Janata Party and the upstart Aam Aadmi Party.
In Uttar Pradesh state, a key battleground that sends 80 MPs to Parliament, voters in an area hit by religious riots last August also went to the polls, including those still living in refugee camps.
Telegraph Group Ltd, AFP
