Gordon Brown will urge fellow world leaders to "make the desirable possible" by showing the statesmanship needed to secure a global climate change deal.
The British Prime Minister is among leaders making short addresses to the UN conference in Copenhagen on Thursday on the eve of the deadline for reaching a worldwide agreement.
Mr Brown has been engaged in a series of meetings with leaders in a bid to secure agreement to proposals he hopes will break the impasse and prevent the summit ending in deadlock.
His contribution from the platform is due to come shortly after one of the most significant of the bi-laterals, with Chinese premier Wen Jiabao.
In what aides said would be a "passionate" appeal for a "final push", Mr Brown will say: "The task of politics is to overcome obstacles even when people say they cannot be surmounted.
"The task of statesmanship is to make the desirable possible, and make ideals real even when critics tell you they are unachievable dreams."
And in a swipe at climate change deniers, he will tell delegates: "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.
"Informed by science, moved by conscience, inspired by common purpose we, the leaders of this world, must say: we will not condemn millions to injustice without remedy, to sorrow without hope, to despair without end."
