Researchers in India and Canada for the Lancet journal said prenatal selection and selective abortion was causing the loss of 500,000 girls a year.

The unusual gender balance in India has been known about for some time.

In most countries, women slightly outnumber men, but separate research for the year 2001 showed that for every 1,000 male babies born in India, there were just 933 girls.

In cases where the preceding child was a girl, the ratio of girls to boys in the next birth was 759 to 1,000.

However, for a child following the birth of a male child, the gender ratio was roughly equal.

Last year the well-known religious leader and social activist, Swami Agnivesh, began a campaign across five northern and western states against female foeticide.