Climate change could shrink the global economy by a fifth unless drastic action is taken, a review is to warn.
The Prime Minister has warned that the international community cannot afford to wait before tackling climate change.
Tony Blair said the Stern report showed the scientific evidence of global warming was "overwhelming" and its consequences "disastrous".
The report, by economist Sir Nicholas Stern, warns that global warming could shrink the global economy by 20%.
But taking action now would cost just 1% of global gross domestic product, economist Sir Nicholas Stern says.
But taking action now would cost just 1% of global gross domestic product, economist Sir Nicholas Stern says.
Without action up to 200 million people could become refugees as their homes are hit by drought or flood, he adds.
Without action, up to 200 million people could become refugees as their homes are hit by drought or flood, he adds.
In the wake of the report, Chancellor Gordon Brown is to promise the UK will lead the international response to tackle climate change.
'No time to wait'
The 700-page Stern Review, published on Monday, says the key to solving the crisis is getting the big polluting countries, such as the US and China, to cut their emissions - through measures including tax, carbon trading and regulation.
"In other words, for every £1 invested now we can save £5 - or possibly more - by acting now," Mr Blair said.
"We can't wait the five years it took to negotiate Kyoto - we simply don't have the time. We accept we have to go further (than Kyoto)."
Sir Nicholas, a former chief economist of the World Bank, told BBC Radio Four's Today programme: "Unless it's international, we will not make the reductions on the scale which will be required."
Sir Nicholas, a former chief economist of the World Bank, told BBC Radio Four's Today programme: "Unless it's international, we will not make the reductions on the scale which will be required."
He added: "What we have shown is the magnitude of these risks is very large and has to be taken into account in the kind of investments makes today and the consumption patterns it has."
He went on: "What we have shown is the magnitude of these risks is very large and has to be taken into account in the kind of investments the world makes today and the consumption patterns it has."
'Global recession'
The increase in the cost to the rich countries for taking on bigger responsibilities would be very manageable Sir Nicholas Stern
Sir Nicholas will say the polluters must be made to "pay the price" for the problems they are causing the planet.
He told the BBC: "The poor countries will be hit earlier and harder and the responsibility of the greenhouse gases currently in the atmosphere is largely the rich countries'. It is only right that the rich countries pay a little more."
He added any cost to rich countries would be "manageable", whereas failure to act could "derail growth".
Prime Minister Tony Blair said the report was the most important one received by the government during his period in office.
He said Britons were keen to lead the way in saving energy, but the problem needed to be tackled globally not just domestically.
Scientific evidence of global warming and its impact was "overwhelming", Mr Blair said.
"Unless we act now the consequences will be irreversible."
"Investment now will pay us back many times in the future - not just in terms of the environment but economically as well."
The report warns unless the world moves to cut green house gases it is heading for a "catastrophic climate change" which would create the worst global recession ever seen.
The Stern Review forecasts that 1% of global gross domestic product (GDP) must be spent on tackling climate change immediately.
The Stern Review forecasts that 1% of global gross domestic product (GDP) must be spent on tackling climate change immediately.
It warns that if no action is taken:
It warns that if no action is taken:
Floods from rising sea levels could displace up to 100 million people
Floods from rising sea levels could displace up to 100 million people
Melting glaciers could cause water shortages for 1 in 6 of the world's population
Melting glaciers could cause water shortages for 1 in 6 of the world's population
Wildlife will be harmed; at worst up to 40% of species could become extinct
Wildlife will be harmed; at worst up to 40% of species could become extinct
Droughts may create tens or even hundreds of millions of 'climate refugees'
Droughts may create tens or even hundreds of millions of "climate refugees"
The study is the first major contribution to the global warming debate by an economist, rather than a scientist.
The study is the first major contribution to the global warming debate by an economist, rather than a scientist.
'Environmental price'
In the wake of the report, Chancellor Gordon Brown is to promise the UK will lead the international response to tackle climate change.
Already Environment Secretary David Miliband is considering a range of taxes designed to change people's behaviour to offset global warming.
He has recruited former US Vice President Al Gore as an environment adviser, while Environment Secretary David Miliband is considering a range of taxes designed to change people's behaviour to offset global warming.
And Mr Brown has recruited former US vice president Al Gore as an environment adviser.
Meanwhile, the Conservative Party said it was examining the possibility of taxing air travel.
BBC environmental analyst Roger Harrabin said some economists say climate science is so uncertain that we should not spend huge amounts now to cut emissions.
However the review says failure to act early could end up costing between 5% and 20% of global GDP and render large parts of the planet uninhabitable with poor nations hit first and hardest.
Africa is likely to be most harmed by climate change and Sir Nicholas says we have a "moral duty" to cut emissions.
HAVE YOUR SAY We need to acknowledge the cost of our actions on the environment Tony, Bristol Send us your views Switching to cleaner energy sources, like wind and solar, can help us avoid the worst of the damage, the report adds.
Green taxes and changing behaviour will help reduce the effect of climate change - but any schemes should encompass the globe, it continues.
Unilateral moves would not be enough, says Sir Nicholas. For example, if the UK shut down all of its power stations tomorrow, the reduction in global emissions would be wiped out in just over a year by increased emissions from China.
'Urgent deal needed'
The review calls on the international community to sign a new pact on greenhouse emissions by next year rather than in 2010/11, when they had planned to agree a successor to the Kyoto agreement on cutting carbon dioxide and other gas emissions.
Poor communities, who have contributed least to climate change, are suffering the most from its effects Tearfund Advocacy Review issues stern warning
Even if immediate action is taken to cut pollution, slow acting greenhouse gases will continue to have an effect on the environment for another 30 years, it adds.
Action groups and development agencies have welcomed the review and urged the government to take action to protect poor countries from the effects of climate change.
"Poor communities, who have contributed least to climate change, are suffering the most from its effects. Current efforts to respond to climate change are simply not urgent enough," said Tearfund Advocacy Director, Andy Atkins.
