Children could die as a result of UK cuts to the overseas aid budget, a senior Conservative backbencher says.
The prime minister is facing a possible Commons defeat after cutting spending of national income on international development from 0.7% to 0.5%.
David Davis - among more than 30 Tories against the move - said some UK-backed schemes have already been cancelled and "morally, this is a devastating thing".
The government's supporters say the cut is temporary, necessary and popular.
The Conservative Party committed to spending 0.7% in its manifesto for the 2019 general election - but ministers say it is hard to justify given record levels of peacetime borrowing during the pandemic.
The cut amounts to almost £4bn, but the government said it will still spend more than £10bn on foreign aid in 2021.
Speaking at the start of a week in which the UK hosts the G7 summit in Cornwall, Mr Davis said: "Historically, I am a critic of aid spending but doing it this way is really so harmful."
He told BBC Radio 4's Today programme if the government were to offer a compromise of a short time frame for their cut to foreign aid spending, "it helps".
But he said "if you're a small child and suddenly you get dirty water, you get an infection from it and you die, temporary doesn't mean much".
Mr Davis added: "If you're going to kill people with this, which I think is going to be the outcome in many areas, we need to reverse those immediately".
Lee Margaret Webster from charity Action Aid told the BBC the cuts had forced a project tackling violence against women and girls in Rwanda to close.
This included shutting down seven shelters providing sanctuary for girls escaping domestic violence, she said.
Former Prime Minister Theresa May is one of the other Tory rebels hoping to achieve a U-turn with an amendment to the Advanced Research and Invention Agency (ARIA) Bill - a planned new law to set up a new agency designed to come up with innovative policy.
It would oblige the agency to make up any shortfall in aid spending if the government were to miss the 0.7% target.
The bill is due to be scrutinised in the Commons on Monday afternoon - but it will be up to Commons Speaker Sir Lindsay Hoyle to decide whether the rebels' amendment should be put to a vote.
BBC politics correspondent Iain Watson says the government is confident the Speaker will rule that the amendment is not sufficiently connected to the bill and therefore will not be debated.
However, he says the Conservative rebels believe Sir Lindsay has not yet made up his mind.
Voting on the bill will begin no later than 21:00 BST.
It is not certain that the Speaker of the Commons Sir Lindsay Hoyle will allow a vote to go ahead later, but if he does, the numbers look very tricky for the government.
First, plenty of backbenchers, and also privately some in the government, think that cutting aid right now is simply the wrong thing to do as a matter of principle.
Next, the commitment to stick with spending 0.7% of GDP was in the Tory manifesto, and MPs don't like having to explain why promises have been broken.
Thirdly, the cast list of those arguing to keep the commitment is full of well known Conservatives, including former prime ministers and cabinet ministers, who know how to get headlines, and how to organise.
There are dozens of former ministers on the backbenches, who may not agree on everything but can agree on this, and are not exactly Mr Johnson's biggest fans.
Lastly, the timing of this vote is awkward indeed. Mr Johnson is about to play host to the leaders of the free world at the G7.
Does starting that week being beaten by his own backbenchers for welching on a promise to the world's poorest sound tempting?
Former Labour Prime Minister Gordon Brown also criticised the cuts, telling BBC Breakfast it was "a life and death issue" and made "no economic or moral sense".
Writing in the Guardian, former International Development Secretary Andrew Mitchell, who is leading the rebellion, said the UK is the only member of the G7 group of advanced economies cutting aid this year - and that "contributing our fair share of aid is essential for a successful G7 summit" this week.
The aid reduction has meant millions of pounds less is being spent on supporting girls' education, reproductive health, clean water, HIV/AIDS, the humanitarian crisis in Yemen and Syria, and hundreds of other projects.
Duty to 'protect our citizens'
Solicitor General Lucy Frazer said the government would return to spending 0.7% levels "when the fiscal situation allows".
She said the UK was a big aid donor but added: "We have a duty to ensure we protect our citizens here as well as those in the rest of the world."
A senior government source told BBC political correspondent Chris Mason that if the amendment were to go through, it would be the equivalent of putting up income tax by a penny for every pound earned.
And ex-Work and Pensions Secretary Esther McVey argued the UK shouldn't "end up going into deeper debt in order to finance other countries".
Writing in the Telegraph, she said the UK should instead help poorer countries "trade their way out of poverty".
"If more and more aid was the solution, large parts of Africa would have escaped all poverty decades ago," she added.
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