LONDON — Prime Minister Boris Johnson of Britain promised on Friday to end direct taxpayer support for fossil fuel projects overseas as soon as possible, in a move designed to help position his country as a global leader in the battle to curb climate change.

The announcement, to be made formally on Saturday at a Climate Ambition Summit convened by the United Nations, Britain and France, follows other recent pledges intended to burnish Mr. Johnson’s environmental credentials and put pressure on other nations to reduce their emissions.

Next year Britain will host the United Nations climate negotiation in Glasgow, a meeting that is seen in Downing Street as an opportunity for the country to demonstrate that Brexit has not diminished its desire to play a leading role on the global stage. Climate change is also a policy where Mr. Johnson expects to find common cause with President-elect Joseph R. Biden Jr. who — unlike President Trump — opposed Brexit, which the prime minister championed.

In a statement the prime minister’s office said that Britain would end financing, aid and trade promotion for new projects overseas to extract or use crude oil, natural gas or the type of coal burned to generate electricity. There will be a few limited exceptions for gas-fired power plants and other projects, within parameters that align with the Paris Agreement on climate change, the government said.