Tony Blair has admitted that he is changing his views on combating global warming to mirror those of President Bush - and oppose negotiating international treaties such as the Kyoto Protocol.
His admission, which has outraged environmentalists on both sides of the Atlantic, flies in the face of his promises made in the past two years and undermines the agreement he masterminded at this summer's Gleneagles Summit. And it endangers talks that opened in Ottawa this weekend on a new treaty to combat climate change.
The U-turn will inevitably bring accusations that he has, once again, sold out to Mr Bush, just at the time that the US President is coming under unprecedented pressure to change his policy in the wake of hurricanes Katrina and Rita. Last week the UK Government's chief scientific advisor, Sir David King, said that global warming might have increased their severity.
Over the past two years Mr Blair has consistently claimed global leadership in tackling what he described as "long term, the single most important issue we face as a global community" and has stressed that it "can only properly be addressed through international agreements". President Bush repeatedly expressed anger at his position.
Sharing a platform with the US Secretary of State, Condoleezza Rice, in New York this month, Mr Blair confessed: "Probably I'm changing my thinking about this", adding that he hoped the world's nations would "not negotiate international treaties".
This contradicts his assertion in a speech a year ago - which drew a private rebuke from the Bush administration - that "a problem that is global in cause and scope can only be fully addressed through international agreement".
It also denies what his ministers claimed to be his main achievement on global warming at Gleneagles. He had succeeded in getting all the leaders except Mr Bush to sign up to negotiating a successor to the Kyoto treaty, and in arranging a meeting between the G8 and leading developing countries to discuss it.
But instead of endorsing agreed limits on the pollution that causes climate change, Mr Blair told this month's meeting at the Clinton Global Initiative that he was putting his faith in "developing science and technology" - precisely Mr Bush's position.
He justified his change of heart by saying that countries would not negotiate environmental treaties that cut their growth or consumption - another of the President's main contentions. But in another speech last April he said it was "quite false" to suppose that environmental protection would inhibit growth.
Last night, Tony Juniper, executive director of Friends of the Earth, called the Prime Minister's volte-face "unbelievable": "Having failed to practise what he preaches, he is now changing his preaching to match his practice."