"Climate change is a defining global social justice issue," Alexander said on Wednesday.

Droughts and heatwaves from Kenya to Australia and southern Europe have been blamed on global warming which is happening already. Six of the first seven years this century were among the seven hottest since reliable records began in 1850, says the World Meteorological Organisation (WMO).

Poor countries will be hit hardest because they have the least resources to cope when crops fail or storms wreak havoc.

Rich countries' efforts to help developing nations are not entirely altruistic, as concern rises that climate change may trigger more conflict, for example over water, and migration.

"By the middle of this century there could be as many as 200 million people forced from their homes because of rising sea levles, heavier floods and more intense droughts. Where will they go?" said Alexander.

"If today's image of climate change is the polar bear tomorrow's could be the AK47."

The extra funding announced on Wednesday was separate from 800 million pounds that Britain last year pledged to support developing countries' fight against climate change, through a World Bank fund expected to be detailed at the Group of Eight leaders' summit in Japan in July.

Japan presented a US$10 billion package last month to help emerging countries tackle climate change.