George Bush is preparing to make a historic shift in his position on global warming when he makes his State of the Union speech later this month, say senior Downing Street officials.

Tony Blair hopes that the new stance by the United States will lead to a breakthrough in international talks on climate change and that the outlines of a successor treaty to the Kyoto agreement, the deal to curb emissions of greenhouse gases which expires in 2012, could now be thrashed out at the G8 summit in June.

The timetable may explain why Blair is so keen to remain in office until after the summit, with a deal on protecting the planet offering an appealing legacy with which to bow out of Number 10.

Bush and Blair held private talks on climate change before Christmas, and there is a feeling that the US President will now agree a cap on emissions in the US, meaning that, for the first time, American industry and consumers would be expected to start conserving energy and curbing pollution.

'We could now be seeing the beginning of a consensus on a post-Kyoto framework,' said a source close to the prime minister. 'President Bush is beginning to talk about more radical measures.'

The move will be seen as part of a wider repositioning of the Bush government after its comprehensive defeat in last autumn's mid-term elections.

A change of heart on the environment was signalled earlier this month when the US administration unexpectedly announced that polar bears were now an endangered species because their habitat in the US state of Alaska had suffered from melting ice sheets caused by global warming. The government is now required to act on threats to the bears' survival. The EU has its own so-called cap and trade scheme, under which industries are given a quota of carbon dioxide emissions: if they exceed the limits, they must pay for extra credits that can be bought from cleaner industries - an incentive to firms to go green.

Downing Street is increasingly confident that the arguments pushed by Sir Nicholas Stern, author of the recent Treasury report on the cost of global warming, that doing nothing will eventually prove more costly than trying to avert catastrophe are now gaining in momentum. However, Stern warned: 'The US will work it out for itself. Nobody will be telling them what to do, and nobody should.'

Downing Street now expects a broad agreement between EU countries on a successor treaty to Kyoto to be thrashed out at the EU spring council, paving the way for an agreement at the G8.

Blair was also told in meetings with senior senators late last year that they would seek to push through measures on global warming which had been repeatedly blocked by the Republicans before the mid-term elections cost Bush's party control of both Houses of Congress.

But another source close to the negotiations warned that Bush had previously appeared to give ground on climate change, only to fail to make real concessions. The best hope could lie with a post-Kyoto deal for 2009, the source said - by which time Bush will be out of office.

Kurt Davies, research director on climate change for Greenpeace USA, said climate change was now expected to be one of the keynotes of the State of the Union address.

'The sands are clearly shifting on climate change for this administration, but there has to be a concrete follow-up,' he said. 'We were shocked last year when he talked about the US being addicted to oil, but then there was no follow-up to that.'