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.

“Climate change is one of the great global challenges of our age, and it is already costing lives and livelihoods the world over. Our actions as leaders must be driven not by timidity or caution, but by ambition on a truly grand scale,” Mr. Johnson said in a statement.

Decisive action today, would “create the jobs of the future, drive the recovery from coronavirus and protect our beautiful planet for generations to come,” he added.

Also on Friday, European Union leaders agreed to cut net carbon emissions over the next decade by 55 percent of their 1990 levels, despite objections from countries like Poland that remain heavily dependent on coal.

Britain, along with other wealthy countries, is an important source of funds for fossil fuel projects abroad, often by providing loans to British companies involved in the work or underwriting loans from British banks. According to the British government, it has supported 21 billion pounds, or about $27.8 billion, of British involvement in overseas oil and gas projects in the last four years.

Both the Organization for Economic Cooperation and Development and the International Energy Agency have called on countries to curb or stop government subsidies for fossil fuel. An independent scorecard estimates Britain’s total public support for fossil fuel projects at $16 billion a year.

Earlier this month Mr. Johnson announced plans to end the sale of new gas and diesel cars within a decade and to change the way people heat their homes. That was part of a program that the British government said would reduce greenhouse gas emissions, by 2030, by at least 68 percent of their 1990 levels, the fastest rate of any major economy.

Environmental groups largely welcomed that push, though some have questioned whether the government would spend enough to realize its goal of net zero carbon emissions by 2050.

Advocacy groups also welcomed the move to end taxpayer support for fossil fuel projects overseas. “The U.K. can start a domino effect of countries ending taxpayer support for fossil fuel projects around the world, hastening the end of the fossil fuel era and raising the chances of success at the Glasgow summit,” Adam McGibbon of Global Witness said in an emailed statement.

Laurie van der Burg, senior campaigner at Oil Change International, called the announcement “a powerful signal that the era of governments propping up deadly fossil fuels with public money is coming to an end,” but cautioned that important questions about the move remain unanswered.

In Britain the opposition Labour Party offered its support too. “Ending our hypocritical position on fossil fuels financing is a basic prerequisite for being a credible host of COP26,” said Ed Miliband, who speaks for the party on business, energy and industrial strategy, referring to the summit in Glasgow.

“Now ministers need to concentrate on an ambitious agreement in Glasgow which meets the goals of the Paris Accord to limit global warming to 1.5 degrees” Celsius, he added.