Contraception advice is crucial to poor countries' battle with climate change, and policy makers are failing their people if they continue to shy away from the issue, a leading family planning expert said on Friday.

Leo Bryant, a lead researcher on a World Health Organization study on population growth and climate change, said the stigma attached to birth control in both developing and developed countries was hindering vital progress.

"We are certainly not advocating that governments should start telling people how many children they can have," said Bryant, an advocacy manager at the family planning group Marie Stopes International, who wrote a commentary in the Lancet medical journal on Friday.

"The ability to choose your family size...is a fundamental human right. But lack of access to family planning means millions of people in developing countries don't have that right," he told Reuters.

Bryant's study of climate change adaptation plans by governments in the world's 40 poorest countries showed that almost all of them link rapid population growth to environmental impact, but only six had proposed steps to tackle it.

"Acknowledgement of the problem is widespread, but resolve to address seems to be very much a minority sport," he said.

Bryant said 200 million women across the world want contraceptives, but cannot get them. Addressing this need would slow population growth and reduce demographic pressure on the environment.

In most countries with good access to birth control, average family sizes shrink dramatically within a generation, he said. But policymakers in rich donor nations are wary of talking about contraception for fear of being accused of advocating draconian ideas like sterilization or one-child policies.

Bryant's comments echo those by the head of Britain's science academy Martin Rees, who told Reuters this month that the stigma holding women back from getting access to birth control must be removed to reduce the impact of rising populations on climate change.

The world's population is forecast to rise by one third to more than 9 billion people by 2050, with 95 percent of this growth in developing countries.

In a study to be published in the WHO Bulletin in November, Bryant and colleagues said that population growth in poorer nations was unlikely to increase global warming significantly, as their carbon emissions are relatively low. But overpopulation combined with climate change would worsen living conditions by degrading natural resources, they said.

Climate change can also not only cause more natural disasters such as storms, but force people to live in areas at risk of floods, drought and disease.

Bryant said health policies targeting family planning must be combined projects to educate people about sustainable farming and land management.

"Then the gains that you make in environmental sustainability in the immediate term are going to be protected in the long term against a rapidly growing population," he said.