Cutting Britain's overseas aid budget risks causing 100,000 otherwise preventable deaths, former international development secretary Andrew Mitchell warned as Boris Johnson faced a growing Tory backlash.
Chancellor Rishi Sunak announced the aid budget will be cut to 0.5% of gross national income in 2021, adding the Government's "intention" is to return it to 0.7% when the fiscal situation allows.
Foreign Office minister Baroness Sugg, whose brief includes sustainable development, has submitted her resignation to Mr Johnson in protest against the cut.
In her resignation letter to Prime Minister Boris Johnson, Baroness Sugg wrote: "It is with sadness that I write to resign from the Government.
"Many in our country face severe challenges as a result of the pandemic and I know the Government must make very difficult choices in response. But I believe it is fundamentally wrong to abandon our commitment to spend 0.7% of gross national income on development. This promise should be kept in the tough times as well as the good.
"Cutting UK aid risks undermining your efforts to promote a Global Britain and will diminish our power to influence other nations to do what is right. I cannot support or defend this decision. It is therefore right that I tender my resignation."
Mr Mitchell led Conservative opposition to the policy in the Commons, and it now looks likely to face a parliamentary showdown when the expected legislation to implement the change is brought forward by the Government.
The 0.7% target is written into law and Mr Johnson's 2019 election manifesto promised to keep it.
Conservative Tobias Ellwood, the Defence Committee chairman, warned China and Russia are likely to extend their "authoritarian influence" as a result of the "vacuum" created by the UK "downgrading" its soft power programmes.
Tory Pauline Latham (Mid Derbyshire) also suggested the cut will hit education for girls and result in "more child marriages, more instances of early child birth, more FGM, more domestic violence".
The UK will be "poorer in the eyes of the world" due to the aid cuts, Conservative former foreign secretary Jeremy Hunt added.
Outside the Commons, the Archbishop of Canterbury attacked the Government for its "shameful and wrong" cuts.
Mr Sunak said the UK's annual aid spend from 2021 is expected to be £10 billion under the temporary cut – compared to the previous figure of £15 billion.
Elsewhere, the Chancellor announced a new £4 billion "Levelling Up Fund" for communities across the country in a Spending Review he said "delivers on the priorities" of the British people.
But Mr Mitchell told the Commons: "As a result of the pandemic here in the UK, 50,000 people have died and we are rightfully moving heaven and earth to prevent more deaths here at home.
"But is (Mr Sunak) aware his proposed breaking of the 0.7% promise and the 30% further reduction in cash will be the cause of 100,000 preventable deaths, mainly among children?
"This is a choice I for one am not prepared to make and none of us in this House will be able to look our children in the eye and claim we did not know what we were voting for."
Mr Sunak said he believes the UK can "still make a difference" to the world's poorest countries under the Government's plans.
He added: "I think the most pressing issue that the developing world faces at the moment is the ability to deliver and deploy a coronavirus vaccine. He will know that we are the largest donor globally to the COVAX Advance Market Commitment, the global initiative which is supporting development countries' access to vaccines."
Mr Ellwood later said: "Downgrading our soft power programmes will lead to vacuums in some of the poorest parts of the world that will further poverty and instability, and is likely to see China and indeed Russia extend their authoritarian influence by taking our place.
"We cannot genuinely claim to be global Britain nor serious about creating post-conflict strategies for countries like Libya and Yemen that could lead to great UK prosperity when our hard power is not matched by our soft power."
But some on the Tory backbenches were supportive, with Philip Davies (Shipley) insisting people in the "real world" will back the cut.
Mr Davies added: "I suspect that the vast majority of the British public won't be asking why has he cut so much, they will probably be asking why are we still spending so much."
But on Twitter, Archbishop of Canterbury Justin Welby wrote: "The cut in the aid budget – made worse by no set date for restoration – is shameful and wrong. It's contrary to numerous Government promises and its manifesto.
"I join others in urging MPs to reject it for the good of the poorest, and the UK's own reputation and interest."
The cut in the aid budget – made worse by no set date for restoration – is shameful and wrong. It's contrary to numerous Government promises and its manifesto.
I join others in urging MPs to reject it for the good of the poorest, and the UK's own reputation and interest.
— Archbishop of Canterbury (@JustinWelby) November 25, 2020
Oxfam chief executive Danny Sriskandarajah said: "Cutting the UK's lifeline to the world's poorest communities in the midst of a global pandemic will lead to tens of thousands of otherwise preventable deaths."
Environmental and aid campaigners warned the move puts the UK's global climate leadership at risk ahead of the United Nations Cop26 summit next year.
Funding to support poorer countries cope with climate change and development cleanly is always a key issue at the international talks.
Greenpeace UK's head of politics, Rebecca Newsom, said: "It will hinder poorer countries' ability to tackle and adapt to the climate emergency, and sour the UK's diplomatic relationships in the run-up to the crucial Glasgow climate conference next year."
Nigel Harris, chief executive of charity Tearfund, said: "The UK is due to host the UN climate talks and G7 summit next year. How can we be seen as a credible global leader if we cut this vital funding?"
The criticism follows interventions ahead of the statement from former prime ministers Sir John Major, David Cameron and Tony Blair, as well as Nobel Prize laureate Malala Yousafzai.