Mr Sarkozy is the first French leader to attend a Commonwealth summit
UK PM Gordon Brown and French President Nicolas Sarkozy have proposed a multi-billion-dollar fund to help developing nations deal with climate change.
Mr Brown said the $10bn (£6bn) fund should also be used to help developing nations cut greenhouse gas emissions.
Both spoke at the Commonwealth summit in Trinidad, the last major world forum before the global summit on climate change in Copenhagen on 7 December.
Many Commonwealth members are island states threatened by rising sea levels.
Mr Sarkozy, with UN chief Ban Ki-moon and Danish Prime Minister Prime Lars Loekke Rasmussen, is there to give weight to any climate change statement.
The topic was the only issue on the Commonwealth summit's agenda for the first day.
The Queen told delegates tackling climate change gave them the "opportunity to lead once more"
Opening the Trinidad meeting, Queen Elizabeth II said the Commonwealth had an opportunity to lead once more on climate change.
"The threat to our environment is not a new concern but it is now a global challenge which will continue to affect the security and stability of millions for years to come," she said.
Mr Brown said half of the $10bn fund should go towards helping developing nations reduce their greenhouse gas emissions and the other half towards helping them adapt to climate change.
James Robbins, BBC diplomatic correspondent
It's a highly unorthodox summit, which may be just what the Commonwealth needed to counter charges it is an irrelevant relic of Britain's imperial past.
This time, key leaders from outside the Commonwealth were invited to the first day - given over entirely to the dominant world issue: climate change.
The UN secretary-general, the French president, and Denmark's prime minister seized the chance of the last major political gathering before December's crucial global negotiations to urge the heads of more than 50 nations here to lead by example and ease deadlocks between the developed and developing world.
If the Commonwealth is a microcosm of the wider world, spanning giant India, rich Britain, Australia and Canada, as well as some of the smallest and most vulnerable island states like the Maldives, then where better to argue that all leaders should prepare to make bold concessions to achieve a better outcome in Copenhagen?
Judging whether or not this Summit really makes a difference may be impossible, but it has certainly raised the Commonwealth's profile, and reminded the world beyond the Commonwealth that, at the very least, huge amounts of political effort are being expended to try to maximise success in Copenhagen.
The first cash would be made available next year, he said, before any emissions deal could take effect.
He is offering $800m from the UK over three years, money that has already been budgeted for.
"What I feel the developing countries need to know is that we are absolutely serious that we would start now," he said, quoted by Reuters news agency.
In separate remarks quoted by AFP news agency, Mr Sarkozy proposed a funding programme of $10bn a year in the years 2010-12, and an "ambitious mechanism" for payments beyond those years.
He did not indicate how much France was prepared to contribute.
The two leaders said the move could encourage developing nations concerned about the economic consequences of reducing emissions to sign up to a climate treaty.
'Pressure' on India
The Commonwealth's 53 nations comprise nearly two billion people, a third of the planet's population.
The leaders are meeting days after pledges by the US and China to limit their greenhouse gas emissions, amid concerns that the Copenhagen meeting could fail to agree substantial cuts.
India has admitted that China's decision to unveil emissions targets two weeks before the Copenhagen summit has put it under pressure.
Made up of former British colonies, dependencies and other territories, plus Mozambique
Founded in 1931
Currently 53 members, with combined population of 1.8 billion
Headed by British monarch, but no allegiance to Crown since 1947
Indian Environment Minister Jairam Ramesh said China's decision was a "wake-up call to India".
Mr Ban said new and positive commitments from several countries over recent days were very heartening.
"My message to all the world leaders has been simple: stay focused, stay committed - and come to Copenhagen and seal a deal," he said.
But in another Commonwealth country, Australia, the government's plans to enact a law for an emissions trading scheme have been thrown into chaos by a revolt within the opposition Liberal Party, whose support is required to pass the bill.
The summit will also discuss Rwanda's entry into the English-speaking club. The Francophone nation has been seeking membership following disagreements with France over events leading up to the 1994 genocide.
The issue is likely to be controversial. The nation's entry bid has received strong backing from some member states.
However, some rights activists are angry that entry would reward a nation they say is guilty of abuses dating back to the 1994 genocide.
Meanwhile the UK has indicated it will try to block Sri Lanka's bid to host the next Commonwealth summit over its handling of the recent war.
A UK government source said Mr Brown had "real concerns about Sri Lanka's bid".
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