The prime minister told an international meeting in New York he was “changing (his) thinking about this”.

“We have got to start from the brutal honesty about the politics of how we deal with it,” he said. “The truth is no country is going to cut its growth or consumption substantially in the light of a long-term environmental problem. To be honest, I don’t think people are going, at least in the short term, to start negotiating another major treaty like Kyoto.”

Blair’s words will undermine the efforts of Margaret Beckett, his environment secretary, who is drawing up plans for just such a new treaty. She is due to fly to Montreal in November to begin talks on the treaty, to take effect when the Kyoto accord ends in 2012.

The prime minister’s comments were made on September 15 at the Clinton Global Initiative, hosted by the former American president at the Sheraton New York hotel. The event was attended by Condoleezza Rice, the US secretary of state, and King Abdullah of Jordan, in addition to Blair.

In his comments, Blair suggested he no longer had faith in global agreements as a way of reversing rising greenhouse gas emissions. Instead he appeared to place his faith in science, technology and the free market — a position that President George W Bush adopted when he repudiated the Kyoto treaty in 2001.

“How do we move forward and ensure that, post-Kyoto, we do try to get agreement?” Blair said. “I think that can only be done by the major players in this coming together and finding a way for pooling their resources, their information, their science and technology.

“The real issue is how do we put these incentives in the system so that the private sector, as well as the public sector, says let’s start getting behind this?” Blair’s suggestion that the answer to climate change lies in the free market has alarmed environmentalists. They were already fearful that the issue was slipping off the agenda of world leaders after this summer’s G8 summit in Scotland.

A communiqué issued by the leaders downgraded climate change from a global “threat” to a “challenge”. It also suggested reducing greenhouse gas emissions, a key element of Kyoto, was no longer an immediate aspiration.

This weekend a Downing Street spokesman declined to comment further on what Blair had said. The Department for Environment, Food and Rural Affairs also declined to comment.

In a speech last week Beckett reaffirmed Britain’s commitment to cut greenhouse gas emissions by 60% by 2050.

Tony Juniper, director of Friends of the Earth, said Blair appeared to be “losing the plot” on climate change.

“Only three months ago at Gleneagles every country except America still supported Kyoto,” he said. “Labour’s credibility on climate change is already collapsing at home and now appears to be disappearing abroad.”

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