Green campaigners supported the first such offer from a world leader and praised its timing two weeks before a climate summit of the 17 biggest developed and developing economies. India said the offer fell short but was something to build on.

U.N.-led talks meant to lead to a new treaty to fight climate change when representatives of 190 countries meet in Copenhagen in December have struggled on disagreement over how far rich countries should fund action in developing countries.

"I propose we take a working figure of around $100 billion per annum by 2020," Brown said in his speech at London Zoo, with a backdrop of emus and wallabies in an arid landscape, hinting at droughts scientists say await Europe without climate action.

"If we are to achieve an agreement in Copenhagen, I believe we must move the debate from a stand-off over hypothetical figures," he told foreign diplomats and public figures.