Gordon Brown demanded more ambition from countries deadlocked in climate change talks today but conceded that it could take up to a year to secure a binding international deal on global warming.
With the Copenhagen negotiations mired in procedural wrangles, the Prime Minister used his keynote speech to appeal for "the highest possible level of ambition".
UK officials conceded today that the process was in "serious difficulty" with time running out for deep disputes among the 192 nations to be resolved before world leaders try to finalise agreement tomorrow.
In a last-ditch bid to end the impasse, Mr Brown told delegates: "My talks this week convince me that while the challenges we face are difficult and testing, there is no insuperable barrier of finance, no inevitable deficit of political will, no insurmountable wall of division that need prevent us from rising to the needed common purpose and ... reaching agreement now.
"In these few days in Copenhagen, which will be blessed or blamed for generations to come, we cannot permit the politics of narrow self-interest to prevent a policy for human survival."
He called on richer states such as the US and European countries to offer the maximum possible reductions in emissions and to pledge not to cut other aid to pay for long-term help for the developing world.
Poorer nations also had to raise their ambitions, he warned, and he said agreement was needed on transparency to ensure commitments were met.
That is a potential sticking point with China, with whose premier Wen Jiabao he will hold the latest of a series of bilateral discussions this afternoon.
But Mr Brown, who has in recent days put a six-month deadline on turning any political agreement signed in Copenhagen into a legally-binding treaty, said twice that time could be needed.
He said: "I come to this, the largest ever global conference, facing the greatest global challenge of our time, to appeal to you to summon up the highest level of ambition and will.
"And the success of our endeavours depends on us forging a new alliance, the first global alliance of 192 - not one bloc against another, not rich against poor - but a new alliance for the preservation of our planet."
He went on: "Informed by science, moved by conscience, inspired by common purpose, we, the leaders of this fragile world, must affirm: we will not condemn millions to injustice without remedy, to sorrow without hope, to deprivation without end.
"The task of politics is to overcome obstacles even when people say they are too formidable.
"And the task of statesmanship is to make the essential possible, to make ideals real even when critics tell you they are impractical and unachievable."
No country was being asked to "suspend" its national interest, he said, "but to advance it more intelligently".
"For nothing matters more to any nation's interest than the fate of the only world we have."
A deal would create green jobs in developed countries such as the UK and give the developing nations the chance to grow without needing a high-carbon economy.
Citing Winston Churchill, he said: "It is not enough for us to do the least we can get away with when history asks that we demand the most of ourselves.
"As one of the greatest of world leaders warned at a different time of peril, 'it is no use saying we are doing our best'.
"Let us demonstrate a strength of resolve equal to the greatness of our cause.
"And let us prove today and tomorrow the enduring truth that is more telling than any passing setback: that what we can achieve together is far greater than whatever we can achieve unilaterally and alone."
Mr Brown is among a succession of leaders addressing the conference today although US president Barack Obama is not due to arrive until the leaders convene tomorrow.
The Prime Minister, who flew to the Danish capital two days ahead of most other major national leaders to prepare the ground for a deal, expressed optimism yesterday that while failure is a possibility, a deal remains possible despite the serious procedural arguments.
Although there has been some progress on finance for developing countries, questions of who should cut emissions by how much and how much aid should be directed to help vulnerable countries cope with the effects of climate change and ensure they develop more sustainably remain unanswered.
Mr Brown told delegates: "Scientific truths know no boundaries of ideology or politics, And no one can honestly deny that without common action, rising sea levels could wipe whole nations from the map.
"And without common action, extreme temperatures will create a new generation of poor, with climate change refugees driven from their homes by drought, climate change evacuees fleeing the threat of drowning, the climate change hungry desperate for lack of food."
Addressing the issue of checks that promises were kept, Mr Brown was careful not to inflame tensions with the Chinese, amid reports that one of its officials had suggested anything other than a broad agreement was impossible.
He said there should be "transparency in accounting for both developed and developing countries, including international discussion and without diminishing national sovereignty".
With hope of a binding contract emerging from the talks abandoned some time ago, attention is being focused on the next steps if a political agreement can be reached tomorrow.
The next UN climate change conference is not due until late next year in Mexico, although there have been calls to bring it forward to July.
Mr Brown also appeared to soften the language of one passage hitting out at climate change "deniers".
According to extracts of the speech released yesterday, the Prime Minister was to say: "Hurricanes, floods, typhoons and droughts we have, from time immemorial, thought of as invisible acts of God, we can see clearly now as the visible acts of man."
Today he added a word, to say they were "also the visible acts of man".
But the scale of the task remaining even to produce a workable text for them to debate was underlined by Climate Change Secretary Ed Miliband, who said negotiations were "certainly on a knife edge and in real grave danger".
A highly-frustrated Mr Miliband, who has been leading some of the detailed talks, said: "Basically we've spent today arguing about the shape of the table, the nature of the negotiations.
"Given the very short time there is to go ... that is not a good way to go. It now needs leaders, unfortunately, to come in and move this process forward."
After another day of delays and tense negotiations, Mr Miliband warned that the talks to achieve a new agreement could fail because of disagreements over the process, a scenario which he said would be a "farce".
He said people around the world would be rightly furious if negotiators failed to get a new deal because the talks ran into the ground.
"It will be terrible dereliction of duty not to fail because we couldn't agree on the substance but because we couldn't agree on the process.
"I would be a tragedy if we failed to agree because of the substance. It would be a farce if we failed to reach agreement because of the process.
"People will find it extraordinary that this conference that has been two years in the planning and involves 192 countries, which is such an important thing, such important stakes, is at the moment being stalled on points of order," he said.