Election frontrunner Narendra Modi has acknowledged for the first time that he is married, solving one of the biggest mysteries about the private life of the man tipped to be India's next prime minister.

The 63-year-old is routinely described as a bachelor and is thought to have lived alone in his adult life having come through the ranks of a grassroots Hindu nationalist organisation in which celibacy is expected.

Media reports have described how he walked away from a marriage arranged by his parents when he was a child, but this has never been confirmed by the man himself who has portrayed his single status as a virtue while campaigning.

In an affidavit on Wednesday as he filed his papers to stand as a member of parliament from the Vadadora constituency in western Gujarat state, he acknowledged that he had a wife.

In the column of the affidavit to mention spouses, the chief minister of Gujarat wrote the name "Jashodaben", but he states elsewhere in the filing that he had "no information" about her.

Reports have consistently stated that he was betrothed to a 62-year-old now-retired school teacher called Jashodaben, who gave an interview in February in which she said she didn't "feel bad" about not being part of Modi's life.

The relationship is believed to have never been consummated.

Modi, a hardline Hindu nationalist consistently ahead in the polls, 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.

He has contested four times for seats in Gujarat's regional assembly but has never stated his marital status on the affidavits.

India's mammoth national election, which entered the third of nine phases on Thursday with almost a fifth of the 543 seats in the parliament up for grabs, has been described as a "battle of the bachelors".

Modi's main opponent Rahul Gandhi from the ruling Congress party, which has been in power nationally since 2004, is single.

Unlike in many other democracies, being unmarried is not seen as a handicap in India because Hinduism has a strong tradition of asceticism in which followers renounce worldly pleasures.

Other parts of Modi's personal story remain unclear, particularly a period as a young man when he is believed to have wandered in the Himalayas on a journey of spiritual discovery.