LONDON, Nov 25 (Reuters) - Weather-related disasters have quadrupled over the last two decades, a leading British charity said in a report published on Sunday.

From an average of 120 disasters a year in the early 1980s, there are now as many as 500, with Oxfam attributing the rise to unpredictable weather conditions cause by global warming.

"This year we have seen floods in South Asia, across the breadth of Africa and Mexico that have affected more than 250 million people," said Oxfam's director Barbara Stocking.

"This is no freak year. It follows a pattern of more frequent, more erratic, more unpredictable and more extreme weather events that are affecting more people.

The number of people affected by disasters has risen by 68 percent, from an average of 174 million a year between 1985 to 1994 to 254 million a year between 1995 to 2004.

"Action is needed now to prepare for more disasters otherwise humanitarian assistance will be overwhelmed and recent advances in human development will go into reverse," Stocking said.

Oxfam wants the UN conference on Climate Change in Bali in December to agree a mandate to negotiate a global deal to provide assistance to developing countries to cope with the impacts of climate change and reduce green house gas emissions. (Reporting by John Sinnott, Editing by Elizabeth Piper)

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