Speaking on the sidelines of the summit, Brown said: "What I feel the developing countries need to know is that we are absolutely serious that we would start now" to provide financing.
"What I'm proposing today is a Copenhagen launch fund. It would start in 2010. It would be $10 billion per annum by 2012."
The Commonwealth - a 53-nation group of mostly former British colonies - brings together wealthy and developing countries.
The UN is aiming for a political agreement at the climate talks in Copenhagen that would cover tougher emissions targets, climate financing for poorer nations and the transfer of clean-energy technology.
The troubled talks have run out of time to settle a legally binding treaty after arguments between rich and poor nations about who should cut emissions, by how much and who should pay.
Commonwealth leaders were aiming to issue a strong political statement in favour of fighting global warming that could leverage a successful outcome in the Copenhagen talks.
The sought-after climate treaty, now expected to be adopted as a final text only next year, will replace the Kyoto Protocol that expires in 2012.
Brown believes the fund proposed by him would give the developing world greater confidence and encourage them to act to counter climate change.
The fund would run from 2010 to 2012, increasing to $10bn in the final year, under Brown's plan. His aides could not say how big the fund would be in the first two years.
The UK is prepared to contribute £800 million ($1.31bn) - already set aside in Britain's budget - over three years to the fund, British officials said.
Brown said he believed the EU and the US would also be ready to contribute to the fund.
|Britain would contribute £800 million to a £10bn fund to help poorer countries [AFP]
EU leaders said last month that developing countries will need €100bn ($149bn) a year by 2020 to battle climate change.
Brown said half of the money in his proposed launch fund should go toward helping the poorest countries adapt to climate change.
The other half would help countries deal with deforestation and building new, cleaner energy sources.
Developing countries would have to provide part of the finance for projects to lessen the impact of climate change and payment from the fund would depend on results.
"The more carbon you reduce, the more tons of greenhouse gas saved, the more money any developing country would get from that fund," Brown said.
He said he believed his proposal would help break a "deadlock" over a deal in Copenhagen.