Barack Obama may be forced to delay signing up to a new international agreement on climate change in Copenhagen at the end of the year because of the scale of opposition in the US Congress, it emerged today.
Senior figures in the Obama administration have been warning Labour counterparts that the president may need at least another six months to win domestic support for any proposal.
Such a delay could derail the securing of a tough global agreement in time for countries and markets to adopt it before the Kyoto treaty runs out in 2012.
American officials would prefer to have the approval of Congress for any international agreement and fear that if the US signed up without it there would be a serious domestic backlash.
Stephen Byers, co-chairman of the International Climate Change Taskforce, said: "The Copenhagen climate change talks in December will come at a difficult moment. The timing couldn't really be worse for the Obama administration. It is vital that this is recognised by the international community. If need be, we should be prepared to give them more time - not to let them off the hook and escape their responsibilities, but ensure they are politically able to sign up to effective international action which reflects the scale of the challenge we face."
Byers, a former cabinet minister who has close contacts with senior Democrats in the Obama team, added: "The practical reality is that a delay into 2010 will still leave time for a new international structure to be put in place for 2012 to follow from Kyoto. Such a delay would be a price worth paying to bring the United States into the global effort to tackle climate change."
The White House's new chief science adviser, John Holden, was a member of the climate change taskforce and Todd Stern, one of its advisers, is working with Hillary Clinton at the State Department and will lead negotiations for the US in Copenhagen.
Stern has warned it will be a tall order to get congressional approval before Copenhagen.
Obama has committed the US to reducing its emissions to 1990 levels by 2020, but scientists and European governments insist deeper cuts are needed. Obama has suggested that the US could compensate with swifter reductions in the years beyond 2020. His recent budget proposal calls for reducing US emissions roughly 80% by 2050 over 2005 levels.
The British government view, including that of the energy secretary, Ed Miliband, is that the Obama administration can and will strike a deal at Copenhagen, but officials in Washington fear America may be running out of time. They have even been looking at whether an agreement would be seen as an international treaty requiring a two-thirds majority in Congress, or whether it could be forced through as a presidential executive order.
But the opposition within America is potentially substantial, and might be hardened if Obama looks like he is presenting Congress with a fait accompli.
There are thought to be as many as 15 Democratic senators who represent "rust-belt" states dependent on coal mining, steel production and heavy manufacturing, all big emitters of carbon.
There have also been suggestions that the cost of any climate change legislation may be higher than the $646bn (£444bn) suggested by the Obama administration.
On Tuesday, Obama recommited himself and America to the principle of a "cap and trade" scheme, but said he would try to introduce a regional scheme that would ensure energy prices did not rise uniformly across America.
Stern would prefer to see the US go to Copenhagen with congressional approval, telling a recent symposium: "The optimum would be [climate] legislation that is signed, sealed and delivered. It has been a long time now that countries have been looking for the United States to lead and take action. I think nothing would give a more powerful signal to other countries in the world than to see a significant, major, mandatory American plan."