The virtual meeting will bring together governments, development banks and others to work on solutions to the floods, droughts and extreme heat faced by many developing countries, as well as ways to boost energy access, clean air and smart cities.
In a statement, London said the event would be an opportunity for countries on the climate frontline to "help set the international agenda" ahead of the G7 summit in June and the COP26 U.N. climate talks to be held in Glasgow in November.
People in developing nations being hit the hardest by climate change are suffering "a searing injustice", Alok Sharma, the UK's president for COP26, is expected to say at the event.
"Developed countries have a particular responsibility to support the response of communities which are most vulnerable to climate change," he will add in a speech.
Foreign Secretary Dominic Raab is due to tell ministers from about 35 countries, including in Asia, Africa and Latin America, that a lack of finance is creating barriers to meeting the goals of the 2015 Paris Agreement on climate change.
"Combined with the challenge of recovering from the pandemic, this threatens to set back progress," he will say.
"We need to consider where international systems can do more to deliver urgent climate action."
The meeting comes amid calls from development organisations for Britain to reverse a decision to cut its foreign aid budget temporarily in response to economic pressure from the COVID-19 pandemic.
London has promised the move will not affect 11.6 billion pounds ($15.9 billion) in climate finance it has committed to deliver over the next five years.
But because international development spending and climate aid often overlap or can strengthen each other, cuts to one can lead to setbacks on both fronts, experts said.
That is particularly true as many debt-laden poor nations struggle to respond to the twin COVID-19 and climate crises, with aid cuts likely to undermine already weak resilience to extreme weather and rising seas, researchers and aid workers said.
In a letter ahead of the summit, the heads of nearly 20 groups working on climate change and development warned the cuts would "inevitably harm the most vulnerable in society, pushing huge numbers (back) into poverty" and should be reversed.
Clare Shakya, director of climate change research at the International Institute for Environment and Development (IIED), said Britain's aid reduction would undermine its credibility in encouraging other wealthy nations to step up funding for climate action.
"In the middle of a pandemic, to have health or education services under threat because of one donor suddenly disappearing" can throw overall progress off track, she told the Thomson Reuters Foundation.
"It's incredible that (the UK) could think that isn't a problem," she added.
The Foreign, Commonwealth & Development Office noted Britain would double its climate finance over the next five years.
"We stand by this commitment and are pushing other developed countries to follow our lead," it said in emailed comments.
The letter from NGOs also urged Britain to appoint a champion to prioritise efforts to tackle losses and damage caused by climate change, and called for a greater share of climate finance to go to efforts to adapt to climate impacts.
That work currently receives only about a fifth of climate funding for developing countries.
The U.N. chief has called for adaptation efforts - to protect people, cities and infrastructure from climate change - to receive half of international flows.
A paper from the Center for Global Development this month found that almost half of about $79 billion in climate finance provided by wealthy countries in 2018 came from existing aid spending and was not "new and additional", as promised.
Ahead of the meeting, Harjeet Singh, ActionAid's global lead on climate change, said governments could "no longer ignore the soaring costs of the climate crisis in the global south".
He cited the example of Mozambique, which had to borrow from the International Monetary Fund to pay for its response to two powerful cyclones in 2019 and is now struggling to help "hungry and desperate" communities rebuild their lives.
"Urgent debt cancellation to enable countries to recover from the COVID-19 health and economic crisis and build climate resilience is also vital," he said in a statement.
Wednesday's meeting is not expected to yield pledges of fresh finance from the donor countries that will attend, which include the United States, Italy, Japan, Norway and France.
The aim is rather to build consensus on practical actions and solutions to better respond to climate impacts and alleviate fiscal pressure so developing countries can better address climate change.
Ministers will also consider ideas to improve the quantity, quality and access to climate finance for vulnerable countries and communities.
Ahead of the event, the UK government announced half a million pounds ($685,000) in funding to develop high-quality voluntary carbon markets, which it said could increase finance flows to where they are most needed.
IIED's Shakya said she hoped the summit would lead to the establishment of a taskforce and plan to push key finance shifts ahead of COP26, including helping poorer nations access major international climate funds more easily.