Billions will be hit by flood and famine, warns the UN

Delivering its bleakest assessment of the problem, the UN added that up to 30 per cent of species could be extinct if temperatures continued to rise.

Agreement on the wording of its report came after an all-night session of tense negotiations in Brussels.

But scientists accused the world's largest producers of harmful carbon dioxide emissions - including the United States, Russia and China - of watering down the conclusions.

Key sections were deleted and scientists confronted government negotiators whom they feared were toning down their findings.

The UN's top climate official Yvo de Boer admitted the report was weakened by the deletion of some elements but insisted it 'will send a very, very clear signal' to governments.

The 21-page document summarising the work of 2,500 scientists from 130 countries warned that without swift action up to 3.2billion faced water shortages.

At the other extreme, the Intergovernmental Panel on Climate Change said that flood waters could claim up to seven million victims in Tokyo and New York by 2080.

"It is the poorest of the poor in the world, even in prosperous societies, who are going to be the worst hit," said Rajendra Pachauri, IPCC chairman. "This does become a global responsibility."

The panel's detailed look at the potential impact on communities if nothing is done aims to galvanise politicians into a united response.

Climate change has already started with wilder weather patterns and increased temperatures, it said.

Billions of people across every continent will be affected, it claimed.

The panel said that by 2020, up to 250million people in Africa would suffer water shortages. A temperature rise of up to 2 degrees Centigrade from 1990s averages would see one third of the earth's species displaced or wiped out.

Disappearing glaciers in the Himalayas would disrupt water supplies, affecting billions across India, China, Nepal and Bhutan.

In Europe, Alpine glaciers would disappear while heatwaves would become common in Britain.

The speed of the changes was brought home by leading British scientist Martin Perry, co-chairman of the panel.

He said he no longer put away his lawnmower for the winter as the grass grew all year round.

The report will be presented to the G8 summit of the world's richest nations in June when the EU is expected to step up pressure on the U.S. to act on fossil fuel emissions.