These papers - part of the Bush administration's submission to the G8 action plan for Gleneagles next month - show how the United States, over the past two months, has been secretly undermining Tony Blair's proposals to tackle climate change.

The documents obtained by The Observer represent an attempt by the Bush administration to undermine completely the science of climate change and show that the US position has hardened during the G8 negotiations. They also reveal that the White House has withdrawn from a crucial United Nations commitment to stabilise greenhouse gas emissions.

The documents show that Washington officials:

· Removed all reference to the fact that climate change is a 'serious threat to human health and to ecosystems';

· Deleted any suggestion that global warming has already started;

· Expunged any suggestion that human activity was to blame for climate change.

Among the sentences removed was the following: 'Unless urgent action is taken, there will be a growing risk of adverse effects on economic development, human health and the natural environment, and of irreversible long-term changes to our climate and oceans.'

Another section erased by the White House adds: 'Our world is warming. Climate change is a serious threat that has the potential to affect every part of the globe. And we know that ... mankind's activities are contributing to this warming. This is an issue we must address urgently.' The government's chief scientific adviser, Sir David King, has dismissed the leaking of draft communiques on the grounds that 'there is everything to play for at Gleneagles.' However, there is no doubt that many UK officials have become exasperated by the Bush administration's refusal to accept the basic principle that climate change is happening now and is due to man's activities.

Earlier this month, the senior science academies of the G8 nations, including the US National Academy of Science, issued a statement saying that evidence of climate change was clear enough to compel their leaders to take action. 'There is now strong evidence that significant global warming is occurring,' they said.

It is now clear that this advice has been completely ignored by Bush and his advisers. 'Every year, it (local air pollution) causes millions of premature deaths, and suffering to millions more through respiratory disease,' reads another statement removed by Washington.

Washington also appears to be unsympathetic towards the plight of Africa, the other priority singled out by Blair for the G8 Summit in Gleneagles.

The documents reveal how the Bush administration has pulled out of financial pledges to fund a network of regional climate centres throughout Africa which were designed to monitor the unfolding impact of global warming.

'Africa, Asia-Pacific and the Arctic are particularly vulnerable to climate variability and are starting to experience the impacts,' reads another excerpt rejected by the US.

Other crucial schemes ditched by the US include the Clean Development Mechanism (CDM) set up to help developing states develop economically while controlling greenhouse gas emissions.