UPDATE 3-Rich must pay $100 bln yearly on climate-UK's Brown

* Rich nations should pay $100 bln to help poor by 2020

* First offer from world leader on rich nation fund

* Seen timely ahead of G8 and climate summits in July

* India says Brown's suggested figure is not enough (Adds Kofi Annan and Oxfam quotes)

By Gerard Wynn

LONDON, June 26 (Reuters) - Developed countries must fund a $100 billion a year fight against climate change in the developing world by 2020, British Prime Minister Gordon Brown said on Friday.

Green campaigners praised the first such offer from a world leader and 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 year 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.

Green groups showed rare, collective enthusiasm after four years of halting global progress to agree a successor to the Kyoto Protocol after 2012.

"The Prime Minister's proposal could give a welcome kickstart to negotiations," said Oxfam International's Tim Gore.

And Kofi Annan, former U.N. secretary-general and president of Geneva-based Global Humanitarian Forum, said the significance of Brown's statement should not be underestimated.

"The world needs this kind of leadership," he added.

FALLING SHORT