Green groups slammed the deal, although they privately acknowledged that Bush has shifted his position substantially from a time when he denied the science of climate change.

Friends of the Earth’s international climate campaigner, Tom Picken, accused G8 leaders of an “elaborate smokescreen” to try to fool the world they were showing international leadership on global warming.

“Setting a vague target for 42 years’ time is utterly ineffectual in the fact of the global catastrophe we all face. Urgent action is needed to tackle climate change and spiralling energy prices caused by our addiction to increasingly expensive and insecure fossil fuels.”

The EU commission said any mention of mid-term goals was an advance from last year when the G8 agreed only to “seriously consider” a goal of halving emissions by mid-century.

Yvo de Boer, head of the UN climate change secretariat, said the G8 deal had positive elements, but warned: “What I find lacking is any kind of language on where industrialised nations, G8 nations, want their emissions to be in 2020 and I think that is critical to making progress in the negotiations.”

US sources said huge challenges remained including an agreed framework to measure carbon reductions, a mechanism to drive down carbon emissions such as an international carbon trading scheme, the scale of the contributions to be required from differing developing countries, and whether targets could be set on economic sectors instead of countries. Populous developing countries such as India claim their per capita emisisons are tiny in comparison with the US. Methods also have to be found to bring aviation and maritime emissions within the scheme.

British officials were pleased that there will also be a new push to set international benchmarks on biofuels, a move that could require US corn producers to scale back on production for biofuels.

Oxfam described the deal as little more than a stalling tactic. They claimed much of the money from the multilateral banks was simply transferred from aid programmes.

Environmental campaign group WWF said: “The G8 are responsible for 62% of the carbon dioxide accumulated in the Earth’s atmosphere, which makes them the main culprit of climate change and the biggest part of the problem.”

Marthinus van Schalkwyk, South African minister of environmental affairs and tourism, said: “As it is expressed in the G8 statement, the long term goal is an empty slogan. To be meaningful and credible, a long-term goal must have a base year, it must be underpinned by ambitious mid-term targets and actions.”