Leaders of the world's biggest emitters of carbon dioxide, including Britain, have agreed to seek measures to limit global warming to 2C.

However they failed to seal a deal on reductions in greenhouse gases.

The declaration, which is the first time America, China and India have all put their names to a target for limiting climate change, came at the G8 summit in Italy which also saw agreement on renewed efforts to conclude a key global trade deal by 2010.

Meanwhile, Prime Minister Gordon Brown indicated that he is ready to put reductions in Britain's Trident nuclear deterrent on the table for negotiation as part of a multilateral deal at a non-proliferation summit called by US President Barack Obama for next spring.

Mr Brown said that he will set out within the next few days specific recommendations for a future non-proliferation framework, which could see nuclear-armed states reduce their own arsenals and offer help with the development of civil atomic energy capacities, in return for non-nuclear states promising not to try to get the bomb.

He told reporters at the L'Aquila summit: "Iran is attempting to build a nuclear weapon. North Korea is attempting to build a nuclear weapon. We have got to show we can deal with this by collective action.

"Unilateral action by the United Kingdom would not be seen as the best way. What we need is collective action by the nuclear weapons powers to say that we are prepared to reduce our nuclear weapons, but we need assurances also that other countries will not proliferate them. And we need new kinds of assurances to prevent a situation such as we have got in Iran emerging in exactly the same way again."

Wednesday's agreement on climate change was reached at a meeting of the Major Economies Forum, chaired by Mr Obama, which brought together the G8 countries Britain, the US, Germany, France, Italy, Canada, Japan and Russia with emerging economic giants China, India, Brazil, Mexico and South Africa as well as major greenhouse gas emitters the EU, Australia, South Korea and Indonesia.

A communique issued after the talks recognised the scientific consensus that global warming should not be allowed to exceed 2C above pre-industrial levels.

And the signatories agreed to work between now and the United Nations climate change conference in Copenhagen this December "to identify a global goal for substantially reducing global emissions by 2050".

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