The world must act now to curb climate change, as doing nothing will cost more long-term, UK officials have said. British government official and former World Bank chief economist Sir Nicholas Stern said pursuing alternative energy made economic and environmental sense. He was addressing a closed-door meeting in Mexico of representatives of 20 of the world's most-polluting nations.

The two-day gathering hopes to reach agreement on ways to meet future energy demands while cutting emissions.

The meeting in Monterrey is the latest round of talks on the climate action plan decided upon at the G8 Gleneagles Summit last year. Ministers from G8 nations are joined at the event by representatives from the emerging economies of China, India, Brazil, and South Africa. Organisers hope the meeting will be able to make progress on a number of issues, including:

* economic challenges of tackling climate change

* alternative low-carbon technologies

* level of investment from public and private sectors

* "road map" for a low-carbon future

British Environment Secretary David Miliband quoted findings reached by Sir Nicholas in his report, which was commissioned by the UK government. "He shows that the longer action is delayed, the more expensive it is," Mr Miliband said. "What he says is that... it is imperative we take action to prevent further climate change because the economic costs - never mind the human costs and the costs to the environment - will far outweigh the costs of mitigation."

The report, regarded by experts as the most authoritative assessment yet of the costs of climate change, will argue that the developed economies, led by the US, face far higher costs if they do not act. Sir Nicholas gave a private briefing on his review to a meeting of
leaders of the world's 20 most polluting nations in Mexico yesterday. The report is said to be framed so that the Bush administration
recognises it will not cost the earth to solve climate change, but will cost the earth literally and financially if it does not.

The report spells out the extra costs of dealing with longer and stronger heatwaves, droughts, rainfall and floods that are already being
observed and are expected to become more extreme.

Other economic costs include greater harvest losses, a rise in the spread of tropical diseases, greater soil erosion and less certainty in
energy supplies.

Although there will be some economic "winners", the report is said to have stated that many poor countries will suffer serious food shortages
and could see large-scale migrations of people.

The presentation comes days after PricewaterhouseCoopers issued a report stating that it will cost $1 trillion to curb emissions of climate-warming gases over the next generation. The Stern review is regarded as a much more rigorous piece of work.

The report is seen as vital ammunition for Tony Blair and Mr Brown as they argue that world leaders need to agree the elements of a future
framework before the current Kyoto agreement on carbon emissions expires in 2012. Mr Bush and the US Senate refused to sign up to Kyoto, arguing the US could not afford to do so if the big emerging economies, notably China and India, were not part of the process.

Also at the meeting in Monterrey, Claude Mandil, head of the International Energy Agency (IEA), presented the findings of extensive research carried out by the agency. Mr Mandil told the BBC that the technologies needed to cut emissions for the foreseeable future already exist. However, he warned that investment in new low-carbon technologies was needed now - otherwise a fresh generation of inefficient, carbon intensive power stations would become locked into the global energy mix. But he said that he was not optimistic that there was a political will to deliver the necessary support, and that there was "a huge gap between words and deeds".

The Ministerial Dialogue on Climate Change, Clean Energy and Sustainable Development was created by the UK when it held the presidency of the G8 in 2005. One of the dialogue's aims was to attempt to reach an informal agreement between industrialised and developing nations on a long-term strategy to cut emissions.

The world's biggest polluter, the US, has not ratified the UN's Kyoto Protocol - the international agreement on reducing nations' greenhouse gas emissions. President Bush rejected it, saying it would harm the US economy and fail to deliver any meaningful reductions. Emerging economies, led by China, argued that if the world's richest nation was not part of the Kyoto targets, it was unfair to expect developing nations to be subject to legally binding limits.

Campaigners hope the Sir Nicholas' findings will help deliver a consensus among the big polluters. "We are urging the G8 not to miss another opportunity to take action in favour of the poorest people of the world, who are already struggling to cope with the effects of climate change," said Rachel Roach, a climate change policy adviser for the aid charity Tearfund. But she added: "Unfortunately, it may well be that this week's meeting is another case of lots of talk but little action."