Nearly 63 million women are "missing" from India's population due to foeticide, disease, neglect, or inadequate nutrition, a government survey says, adding that more than two million women disappear every year.

The survey, released on Monday, pointed out the phenomenon of "son preference" among Indians that has created an estimated 21 million "unwanted" girls.

"Indian parents often continue to have children till they have the desired number of sons," it said.

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"Families that have sons are more likely to stop having children than families where a girl is born. This is suggestive of parents having children until they have as many sons as they want."

Rebecca Reichmann Tavares, a former India representative at the United Nations Entity for Gender Equality and the Empowerment of Women, also known as UN Women, said: "the Indian society has been aware of this issue for some time".

"It is against the law to determine the sex of a foetus, but it is still widely practiced. And we find that even in the states where people are more educated and have higher incomes, the practice is more widespread," Tavares said.

"It really goes to show that economic development and higher level of education are not enough to promote or ensure gender equality. Even having a legal and policy system that has done everything to ensure legal rights for women and for girls, has not been enough."

The survey comes as the sex ratio in India worsened over the years despite government campaigns to bring gender parity.

Prime Minister Narendra Modi launched Beti Bachao, Beti Padhao [Save girls, Educate girls] initiative in 2015 in a sign the government prioritised women empowerment.

"I do believe of course that the government is committed to addressing this issue and it is very seriously committed," Tavares said.