Families are going hungry and girls are missing school because of the UK's cuts to foreign aid spending, dozens of charities and academics have warned.
Charities including Oxfam and ActionAid UK said they had caused "devastation" that could undermine the UK's credibility at the upcoming G7 summit.
In a letter to Boris Johnson, they said there was "no justifiable economic need" for cuts of almost £4bn a year.
The government says it will still spend more than £10bn on foreign aid in 2021.
Health Secretary Matt Hancock told BBC One's Andrew Marr Show it was a "temporary" reduction, which was "entirely reasonable" given that the pandemic had caused a "once-in-300-year economic interruption".
The letter comes ahead of a potential Commons vote on Monday when more than 30 rebel Tory MPs, including former Prime Minister Theresa May, hope to reverse the cuts.
Last year, ministers decided to reduce this year's spending on overseas aid from 0.7% of national income to 0.5%, saying it was hard to justify, given that the UK faces record levels of peacetime borrowing to cope with the pandemic.
But charities including Save the Children, WWF UK and Cafod said the reduction had already led to the closure of feeding centres and clinics - and forced the cancellation of projects including water sanitation and training for healthcare workers.
The letter - signed by more than 1,700 academics, charities and business leaders - was shared with the BBC as the UK prepares to welcome leaders from the G7 group of major advanced economies to a summit in Cornwall.
"While other G7 countries have stepped up their aid budget, the UK is the only one to have rowed back on its commitments," it said.
The letter added that doing this during the pandemic was a "double blow to the world's poorest communities".
What is foreign aid and who does it help?
Foreign aid is resources given from one country to another. It can involve a transfer of things like food or equipment, or even people to provide training and medical help.
In 2020, the UK spent £14.5bn on aid, meeting its 0.7% target, according to provisional data.
Overall, the top five recipients were Pakistan, Ethiopia, Afghanistan, Yemen and Nigeria, with almost all the money going to countries in Africa and Asia.
The government has pledged to restore aid spending to 0.7% "when the fiscal situation allows" - but it has not specified a date.
Ministers cut the spending without changing a 2015 law which made the 0.7% target legally binding.
More than 30 Conservative rebels, including former ministers Damian Green, Stephen Crabb and Johnny Mercer, say they are planning to reverse the cuts on Monday by voting to amend legislation related to a new scientific research agency.
Their amendment - which will need the support of more Tory MPs to pass - would oblige the agency to make up any shortfall from the 0.7% target from January 2022.
It will be up to Commons Speaker Sir Lindsay Hoyle to decide whether the amendment is within the scope of the bill and whether it should be put to a vote.
Anti-poverty campaigner Sir Bob Geldof told the Andrew Marr Show said of the aid cut: "It doesn't make any sense. It doesn't make any sense economically, financially, politically, diplomatically, morally or in the humanitarian consequences of it.
"And I'm very much afraid that something that we're told is temporary will become permanent."
Mr Hancock said: "The decisions that the government has taken around this are entirely reasonable. We face a once-in-300-year economic interruption."
He added that much of the work done by the UK, including distributing vaccines to other countries "at cost", did not officially "count as aid".
Additional reporting by Dulcie Lee.
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