The Prime Minister, speaking at the United Nations in New York, said climate change was a “terrible injustice” which was hitting the world’s most vulnerable nations hardest.

Mr Brown has already proposed a 100 billion dollars (£61 billion) international package to help the world’s developing nations shift to “green” technologies as part of a deal to cut global carbon emissions.

But in opening remarks yesterday to a UN working group on the financing of an agreement, he said that of the 100 billion dollar total, 30 billion dollars would be needed specifically for the poorest and most vulnerable countries.

“Climate change is a terrible injustice - it has been caused almost entirely by the richest countries, but its earliest and most damaging effects will fall - and are already falling - on the poorest,” he said.

The working group was part of a climate change summit called by UN Secretary General Ban Ki moon ahead of crucial climate change talks in Copenhagen in December.

Earlier this week Mr Brown offered to go to the summit himself to try to help push through an agreement amid growing concern among officials that the prospects for a deal are slipping away.

One official travelling with the Prime Minister said the issues involved were so complex, they could only be resolved at the highest level.

“It is too important to be left to officials, it can only be done by leaders,” he said.

Mr Brown will highlight the issue again today when he addresses the UN General Assembly as one of the key challenges facing the world alongside the economy, terrorism, nuclear proliferation and poverty.

The official said while no deal was expected in New York, it was crucial the leaders attending realised just how important the issue was.

“This is not the point at which a deal will be done. This is where the leaders look each other in the eye and say that a deal has to be done,” he said.

Mr Brown warned today that success at Copenhagen could not be taken for granted and urged fellow world leaders to throw their weight behind a deal.

In an article in the New York Times, he stressed the importance of an international climate agreement not only on the environment, but also on the prospects for economic growth, jobs and prosperity.

“Progress is too slow and a deal now hangs in the balance,” warned Mr Brown. “But failure will increase the threat not only of humanitarian and ecological catastrophe but also of economic decline.

“Investment in energy efficiency and low-carbon energy sources will help drive economic growth over the next decade - as well as reduce dependence on imported oil and enhance energy security.

“Millions of jobs stand to be created as this investment expands - the low-carbon sector is now larger than defence and aerospace combined. But it is vital that we give confidence to such investment through a new international climate agreement.

“This will not be possible without the co-operation of developing countries. For this reason, Britain has suggested a program of 100 billion US dollars a year by 2020, financed by wealthier countries and the private sector, to help poorer nations develop low-carbon economies.

“We must move toward resolving the issues that remain before Copenhagen. If it is necessary to secure agreement, I will personally go to Copenhagen to achieve it. I will be urging my fellow leaders to do the same.”