Brown will tell them: "In every era there are only one or two moments when nations come together and reach agreements that make history, because they change the course of history. Copenhagen must be such a time. There are now fewer than 50 days to set the course of the next 50 years and more.

"If we do not reach a deal at this time, let us be in no doubt: once the damage from unchecked emissions growth is done, no retrospective global agreement in some future period can undo that choice. By then it will be irretrievably too late."