Ivan Rogers, Mr Blair's principal private secretary, told Sir David King, the Prime Minister's chief scientist, to limit his contact with the media after he made outspoken comments about President George Bush's policy on climate change.

In January, Sir David wrote a scathing article in the American journal Science attacking Washington for failing to take climate change seriously. "In my view, climate change is the most severe problem we are facing today, more serious even than the threat of terrorism," he wrote.

Support for Sir David's view came yesterday from Hans Blix, the former United Nations chief weapons inspector, who said the environment was at least as important a threat as global terrorism. He told BBC1's Breakfast with Frost: "I think we still overestimate the danger of terror. There are other things that are of equal, if not greater, magnitude, like the environmental global risks."

Since Sir David's article in Science was published, No 10 has tried to limit the damage to Anglo-American relations by reining in the Prime Minister's chief scientist.

In a leaked memo, Mr Rogers ordered Sir David - a Cambridge University chemist who offers independent advice to ministers - to decline any interview requests from British and American newspapers and BBC Radio 4's Today .

"To accept such bids runs the risk of turning the debate into a sterile argument about whether or not climate change is a greater risk," Mr Rogers said in the memo, which was sent to Sir David's office in February. "This sort of discussion does not help us achieve our wider policy aims ahead of our G8 presidency [next year]." The move will be seized on by critics of Mr Blair's stance over the Iraq war as further evidence that he is too subservient to the Bush administration. It will also be seen as an attempt to bolster the Prime Minister's case for pre-emptive strikes to combat the threat of international terrorism, which he outlined in a speech on Friday.

Sir David, who is highly regarded by Mr Blair, has been primed with a list of 136 mock questions that the media could ask if they were able to get access to him, and the suggested answers he should be prepared to give. One question asks: "How do the number of deaths caused by climate change and terrorism compare?" The stated answer that Sir David is expected to give says: "The value of any comparison would be highly questionable - we are talking about threats that are intrinsically different."

If Sir David were to find himself pushed to decide whether terrorism or climate change was the greater threat, he was supposed to answer: "Both are serious and immediate problems for the world today." But this was not what Sir David said on the Today programme on 9 January when the Science article was published.

Asked to explain how he had come to the conclusion that global warming was more serious than terrorism, Sir David replied that his equation was "based on the number of fatalities that have already occurred" - implying that global warming has already killed more people than terrorism.

The leaked memo came to light after a computer disk was discovered by an American freelance journalist, Mike Martin, at the annual meeting of the American Association for the Advancement of Science in Seattle, where Sir David gave a lecture.

"The disk was lying on the top of a computer in the press room and I popped it into the machine to see what was on it," said Mr Martin, whose own article is published on the ScienceNow website, an online service operated by Science.

Mr Rogers' memo, written a few days before the Seattle conference, was aimed at limiting his exposure to questions from US and British media. While in Seattle, Sir David sat on a panel of scientists at one carefully stage-managed press conference, but his press office said he was too busy to give interviews afterwards to journalists.

Lucy Brunt-Jenner, Sir David's press officer, said she could not comment on internal government documents but said it would be wrong to suggest that Sir David was in any way muzzled. "Sir David had a press conference and he was available to the media at three times," Ms Brunt-Jenner said.

The document predicts that abrupt climate change could bring the planet to the edge of anarchy as countries develop a nuclear threat to defend and secure dwindling food, water and energy supplies. The threat to global stability vastly eclipses that of terrorism, say the few experts privy to its contents. 'Disruption and conflict will be endemic features of life,' concludes the Pentagon analysis. 'Once again, warfare would define human life.'

The findings will prove humiliating to the Bush administration, which has repeatedly denied that climate change even exists. Experts said that they will also make unsettling reading for a President who has insisted national defence is a priority.

Climate change 'should be elevated beyond a scientific debate to a US national security concern', say the authors, Peter Schwartz, CIA consultant and former head of planning at Royal Dutch/Shell Group, and Doug Randall of the California-based Global Business Network. An imminent scenario of catastrophic climate change is 'plausible and would challenge United States national security in ways that should be considered immediately', they conclude. As early as next year widespread flooding by a rise in sea levels will create major upheaval for millions.

Last week the Bush administration came under heavy fire from a large body of respected scientists who claimed that it cherry-picked science to suit its policy agenda and suppressed studies that it did not like. Jeremy Symons, a former whistleblower at the Environmental Protection Agency (EPA), said that suppression of the report for four months was a further example of the White House trying to bury the threat of climate change.

Bob Watson, chief scientist for the World Bank and former chair of the Intergovernmental Panel on Climate Change, added that the Pentagon's dire warnings could no longer be ignored. 'Can Bush ignore the Pentagon? It's going be hard to blow off this sort of document. Its hugely embarrassing. After all, Bush's single highest priority is national defence. The Pentagon is no wacko, liberal group, generally speaking it is conservative. If climate change is a threat to national security and the economy, then he has to act. There are two groups the Bush Administration tend to listen to, the oil lobby and the Pentagon,' added Watson.

Already, according to Randall and Schwartz, the planet is carrying a higher population than it can sustain. By 2020 'catastrophic' shortages of water and energy supply will become increasingly harder to overcome, plunging the planet into war. Randall told The Observer that the potential ramifications of rapid climate change would create global chaos. 'This is depressing stuff,' he said. 'It is a national security threat that is unique because there is no enemy to point your guns at and we have no control over the threat.' Randall added that it was already possibly too late to prevent a disaster happening.