There is a cultural preference for male children in India
The number of girls born and surviving in India has hit an all-time low compared with boys, ActionAid says.
A report by the UK charity says increasing numbers of female foetuses were being aborted and baby girls deliberately neglected and left to die.
In one site in the Punjab state, there are just 300 girls to every 1,000 boys among higher caste families, it says.
ActionAid says India faces a "bleak" future if it does not end its practice of cultural preference for boys.
ActionAid teamed up with Canada's International Development Research Centre (IDRC) to produce the Disappearing Daughters report.
More than 6,000 households in sites across five states in north-western India were interviewed and statistical comparisons were made with national census date.
Under "normal" circumstances, there should be about 950 girls for every 1,000 boys, the charity said.
But it said that in three of the five sites, that number was below 800.
In four of the five sites surveyed, the proportion of girls to boys had declined since a 2001 census, the report said.
The research also found that ratios of girls to boys were declining fastest in comparatively prosperous urban areas.
ActionAid suggested the increasing use of ultrasound technology may be a factor in the trend.
The document says that Indian woman are put under intense pressure to produce sons, in a culture that predominantly views girls as a burden rather than an asset.
It says many families now use ultrasound scans and abort female foetuses, despite the existence of the 1994 law banning gender selection and selective abortion.
The charity also blames other illegal practices - such as allowing the umbilical cord to become infected - for the growing gender imbalance.
"The real horror of the situation is that, for women, avoiding having daughters is a rational choice. But for wider society it's creating an appalling and desperate state of affairs," Laura Turquet, women's rights policy official at ActionAid said.
"In the long term, cultural attitudes need to change. India must address economic and social barriers including property rights, marriage dowries and gender roles that condemn girls before they are even born.
"If we don't act now the future looks bleak," Ms Turquet said.
Some 10 million female foetuses have been aborted in India in the past 20 years, the British medical journal the Lancet has said.