The policy will apply to oil, natural gas and coal, but there will be limited exceptions for gas-fired power plants and other projects.
There is no date for the change to come into force, but ministers said they wanted it to be before the UK hosts the COP26 climate conference in November next year.
The prime minister said the reforms would help “protect our beautiful planet for generations to come”.
Experts said the government had “finally seen sense”, but warned the change had to be matched by greater action on climate change at home.
Over the last four years the government has supported £21bn of UK oil and gas exports through trade promotion and export finance.
Friends of the Earth climate campaigner Rachel Kennerley welcomed the announcement but warned there could be “no exceptions” to the policy.
Ministers predicted the announcement would speed up the shift towards green technology and renewable energy, a move they argue will create jobs across the UK.
Ahead of Saturday’s Climate Ambition summit, which will be attended by at least 75 world leaders, Mr Johnson said: “Our actions … must be driven not by timidity or caution, but by ambition on a truly grand scale.”
Business secretary Alok Sharma said the commitments outlined at the conference would “show that we are entering a new age – one of increased climate ambition and real action”.
Laurie van der Burg, senior campaigner at Oil Change International, said the government had not created “a gold standard, as that would require a clear cut end to domestic fossil fuel subsidies estimated at over $14bn [£10.5bn] per annum, investments in gas power and in financial intermediaries that continue to invest in fossil fuels. Nevertheless, this is a massive, albeit long overdue step in the right direction and a win for UK campaigners and activists that have fought for this for years”.
Dr Alison Doig, from the Energy and Climate Intelligence Unit (ECIU) group, said: “The UK government has finally seen sense, that we need to match our action on climate change at home with an equivalent effort overseas.”
Dr Ruth Valerio, from international charity Tearfund, said: “Around the world, the climate crisis is destroying people’s homes, health and livelihoods, so it’s absolutely right for the government to scrap support for the dirty fuels driving this destruction.”
Labour’s shadow energy secretary Ed Miliband said that “ending our hypocritical position on fossil fuels financing is a basic prerequisite for being a credible host of COP26. 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”.
Ahead of the online UN summit, the European Union has outlined a new target to tackle greenhouse gases, pledging to cut emissions by “at least” 55 per cent by 2030 on 1990 levels.
The summit will hear from leaders from many of the world’s major economies including the EU, China, Japan and Canada.
A consultation on the UK’s government’s pledge to end support for fossil fuel projects overseas will be launched at the conference.
It is expected to conclude by 8 February.
