* 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.

The $100 billion figure fell far short of what many developing countries have called for. For example India has suggested that developed countries should provide 1 percent of national wealth, or GDP, and was unimpressed by Friday's offer.

"It's just a drop, but at least somebody has said something at last," said Pradipto Ghosh of New Delhi-based The Energy and Resources of Institute (TERI), and a member of the climate panel of Prime Minister Manmohan Singh.

The 1 percent GDP number was an "obviously fantasy figure", a senior British government official told reporters on Friday.

Brown also said a climate fund must not simply divert rich countries' existing commitments to aid overseas development.

Up to a tenth of such existing promises could be used, he proposed, where steps met both development and climate goals, for example boosting drought resistance and food yields.

Campaigners especially supported his suggestion that the $100 billion fund could be partly raised from international aviation and shipping, for example from taxing or including these sectors in emissions trading markets.

Ships and planes are exempt from carbon cuts under Kyoto. Brown also backed a Norwegian proposal to levy a charge on national emissions rights for rich countries under a new pact.

Britain wants a Copenhagen deal to commit to limit global warming to no more than 2 degrees Celsius, for exampl by setting a goal for global greenhouse gases to stop rising by 2020. (Additional reporting by Krittivas Mukherjee in New Delhi; Editing by Louise Ireland)