THE key to preventing a near-doubling of global population by 2050 is to enable women in developing nations to choose the size and spacing of their families, according to the United Nations population chief.

Thoraya Ahmed Obaid, who heads the the United Nations Population Fund, said research showed about 235 million of the world's women would like to cap their family sizes but had no access to family planning.

Rejecting approaches such as China's One Child policy, she said that slowing down future population growth could be achieved only through voluntary means.

''If these 235 million can plan their family, they all want smaller families. This means it will slow population growth,'' she said.

''When population growth is slowed that will lead to smaller total population, which will have a lighter footprint on environment (and) climate change.''