Gordon Brown has outlined plans to help developing countries
Gordon Brown is "unequivocal" about the scientific case for action against climate change, No 10 has said, ahead of the forthcoming Copenhagen summit.
Claims that scientists manipulated data on global warming have sparked a row over its origins and the basis for reducing man-made carbon emissions.
But Mr Brown said the climate was the "greatest challenge" facing the world.
The UN climate summit, at which leaders are under pressure to agree binding targets, is a "turning point", he said.
Mr Brown is one of a host of world leaders, including US President Barack Obama, who have agreed to attend the summit.
Both the Conservatives and Lib Dems have spoken of the importance of the event reaching a binding global agreement to curb emissions.
Thousands of people are expected to attend a march in London on Saturday calling for an ambitious deal at the summit.
In recent weeks, hopes of what can be achieved in Copenhagen in terms of emission targets and financial support for the poorest countries to face up to the climate threat have taken a knock.
Speaking in London, Mr Brown said policymakers were only "halfway there" in achieving their goals for the summit.
"This perhaps the greatest challenge we face as a world and this is the turning point that can either work for us or it can fail," he said.
"This is one of the great endeavours of our time - to bring the world together to deal with the problem that has been caused essentially by the richest countries but is now affecting some of the poorest countries in the world. If we do not act, all of us are going to be worse off."
Momentum towards a deal in Copenhagen has been hit by a row over the scientific evidence underpinning the arguments over global warming.
Last month hundreds of messages between scientists at the University of East Anglia's Climatic Research Unit (CRU) and counterparts in other countries appeared on the internet along with other documents.
Some observers allege that one of the e-mails suggested CRU head Professor Phil Jones wanted certain papers excluded from the UN's next major assessment of climate science.
Dr Jones, who has stood aside from his job pending the results of an internal review, strenuously denies this was his intention.
But critics of the scientific consensus have claimed that the e-mails undermine the case that greenhouse gas emissions from human activities are causing global warming.
Asked earlier about the row, the prime minister's official spokesman said Mr Brown "respected people's opinions but the science is clear".
Climate Change Secretary Ed Miliband has described the arguments of so-called "climate change deniers" as "nonsense" and urged people not to heed such "siren voices".
Ahead of the summit, he is taking part in what his advisers are calling the "first ministerial mass phone-call" on the issue, at 1000 GMT on Saturday.
He is encouraging people to contact him by phone, e-mail, Twitter or through his website, to discuss what can be achieved at the summit.
Several UK politicians who accept humanity's contribution to global warming have warned against a knee-jerk reaction in Copenhagen which could damage the economy as it moves out of recession.