Britain is set to pledge hundreds of millions of pounds to a United Nations "green bank" intended to help poor countries prepare for the impact of global warming.
The announcement is expected at a pledging conference in Berlin on November 20, after Prime Minister David Cameron said the UK would "play a very positive part" in supporting the Green Climate Fund.
The timing is awkward for Conservatives as it comes on the same day as the crunch Rochester and Strood by-election, where Mr Cameron's party is struggling to fight off a challenge from Ukip, which argues that Britain is already too generous with overseas aid.
No specific figure has yet been confirmed for the UK's contribution, but energy minister Amber Rudd said earlier this month that Britain will donate "strongly" to the fund.
With American President Barack Obama announcing a 3 billion US dollar (£1.9 billion) contribution during the G20 summit in Australia, the pressure is on Britain to make a commensurate donation, and anything in less than nine figures is not thought likely.
Speaking at the Brisbane summit, Mr Cameron was careful to stress that the money will come from funds already earmarked for international development, and do not represent a new financial commitment.
"Britain has already set aside a substantial amount of money for green climate funds," said the Prime Minister. "All we have to do now is to decide how much of that already-set-aside money to put into this specific fund.
"As ever in these things, Britain will play its part and will play a very positive part.
"I think it's been good on this occasion that some other countries have been encouraged to come forward and put some money down.
"Everybody knows that Britain will play our part because we've already set aside money, so when we make an announcement it won't be new money - it's money already set aside for that purpose."
Conservative backbencher Philip Davies said the Government was "completely on the wrong side" of both overseas aid and climate change.
He said: "I am not entirely sure the by-election will be decided by this decision but both are issues that play into Ukip's hands as they go to show the Government is out of touch with public opinion."
On the UK's forthcoming pledge to the "green bank" from existing international development funds, the Shipley MP said: "I suppose we should be grateful for small mercies that it will come out of the existing budget, given we have the ridiculous policy to spend 0.7% on overseas aid."
Mr Davies said the Government should be focused on energy security for Britain and keeping energy prices as low as possible for families struggling to pay them.