Professor Lord Robert May, the immediate past president of the Royal Society, insists that a declining global population is "a prerequisite" if a sustainable ecological change was to be made.
Lord May said a priority was educating and empowering women, "particularly in those cultures where this is not currently the case".
"In my view, religious beliefs or other ideological prejudices prompt some major international organisations to oppose contraception, forbidding distribution of condoms or even advice about fertility control," Lord May said.
Global population growth is predicted to increase to 9 billion by 2050, driven by strong growth in developing countries, while declining birth rates in developed countries create their own inter-generational problems.
However, he insisted that cutting population alone would not address environmental problems what with the everlooming threat of conflicts and mass movements of people as the world fights over limited water supplies and other resources.
"All this rolls together with rapid and continuing advances in information technology, which simultaneously makes things better and worse," he said.
"Better because we can more easily and effectively co-ordinate action, once motivated to do so; worse because in such a global village the massive inequities between groups are clearly exposed."
Lord May was addressing the Lowy Institute in Sydney.