The prime minister has encouraged companies to start spending money on global climate change as he heads to the COP summit in Baku, Azerbaijan.
On the trip to the Caspian coast, Sir Keir spoke to journalists travelling with him.
He was asked if the UK – which the government says is struggling financially – would commit to spending new money on helping developing countries with climate change.
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There have been calls for a $1tn (£777bn) financing agreement to help less wealthy nations reduce emissions and meet other targets.
Sir Keir told journalists his government would honour the monetary promises made at a previous COP summit under the Conservative government.
This current meeting – COP29 – will then “want to look at a future [climate finance] sum taking us through to 2035, but we’re not making UK commitments in relation to that”, he said.
Sir Keir added: “I will be making an argument powerfully that now is the time for the private sector to start paying their fair share in relation to these commitments.”
As part of this, a new “capital market mechanism” will be launched on the London Stock Exchange, with Downing Street hoping it will raise £75bn for green investment over the next decade.
Sir Keir and the government want to take advantage of the surge in green investment as part of their economic growth strategy.
He told reporters he has been clear the “climate challenge” is a “huge opportunity for the UK if we get it right”.
“That is why we have made it one of our missions to have clean power by 2030, and if you look at the inward investment that we have triggered in the last four months, a huge amount of that is on renewables – that is where global investors want to put their investment.
“So I see climate change as an important obligation on which we’ve got to show leadership, but it’s also an incredible opportunity for the UK to get ahead on the world stage, and I am determined we are going to do that.”
Downing Street believes the UK can act as a green financial hub, and use that to bring investment, jobs and growth.
One report suggested the “green economy” grew by 9% last year, while the rest of the UK stagnated, and business urged a pro-green industrial strategy.
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The new government has started approving more projects like onshore windfarms and solar farms.
Increasing growth is key to the plan of Sir Keir and Chancellor Rachel Reeves – but their plans may be difficult to achieve if the economy does not expand at the rate they hope.