China announced yesterday that it intended to slash its greenhouse gas emissions per unit of economic output by 40-45 per cent by 2020, a key requirement for reaching a global deal to tackle climate change at December's Copenhagen conference.

Beijing also said that Wen Jiabao, the premier, would attend the Denmark talks. Analysts said the decisions underlined the seriousness with which Beijing is treating the climate change debate.

The carbon intensity target means reducing the amount of carbon produced per unit of gross domestic product, and is not the same as cutting emissions. Indeed, China's economy will double in size by 2020 at current growth rates and its emissions will be considerably higher even if it meets the new target.

The announcement followed a pledge on Wednesday from the US to cut its emissions by 17 per cent by 2020, provisional on the passage of domestic legislation.

Yvo de Boer, the United Nations' top climate change official, who will steer the Copenhagen talks, said: “The US commitment to specific, mid-term emission cut targets and China's commitment to specific action on energy efficiency can unlock two of the last doors to a comprehensive agreement.”

But he said some crucial issues remained unresolved, particularly the question of developed countries giving financial assistance to the developing world to enable the latter to cut emissions and cope with the effects of global warming.

A senior US official told the Financial Times the US had no imminent plans to table a commitment on finance.

China has not yet set a target date for its carbon emissions to peak, although there has been speculation that Chinese negotiators would make such a commitment at Copenhagen.

The commitment to cutting greenhouse gas intensity is the latest evidence that Beijing is putting in place robust policies, but some pushed Beijing to go further. “Given the urgency and magnitude of the climate change crisis, China needs stronger measures,” said Ailun Yang of Greenpeace China.

One European diplomat in Beijing noted the new target suggested China would achieve smaller gains in energy efficiency over the next decade than it was currently pushing industry to achieve. “They have done a lot of the easier things already.”