Senior Tories want the commitment to spend 0.7% of national income on overseas development restored.
The prime minister could face defeat in the Commons later over government cuts to the UK’s overseas aid budget, at the start of a week in which he hosts the G7 summit in Cornwall.
More than 30 rebel Tory MPs want Boris Johnson to restore a commitment to spend 0.7% of national income on international development.
Ministers reduced it to 0.5% this year – a cut of almost £4bn.
The government’s supporters say the cut is temporary, necessary and popular.
The Conservative Party committed to spending 0.7% in its 2019 manifesto – but ministers say it is hard to justify given record levels of peacetime borrowing during the pandemic.
Former Prime Minister Theresa May is among the rebel Tory MPs 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.
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 eyes of the world are truly upon us. But in this moment Britain is found wanting, because we have removed a foundational piece of our own global leadership,” he has written in the Guardian.
He said cuts were already having a “devastating effect” and would “result in hundreds of thousands of preventable deaths”.
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.
Former Health minister Steve Brine, one of the rebels, said saving 0.2% would have “significantly less” impact on the UK government’s deficit than it would on “those missing out” on the aid overseas.
“I hear people say, and there will be many in my constituency who will say this too, that charity begins at home,” he told BBC Radio 4’s Westminster Hour.
“And I always say, ‘yes, it does’. But it doesn’t end there.”
The government has promised to restore spending to the 0.7% level “when the fiscal situation allows” – but has not specified a date.
Mr Brine is among the rebels open to a compromise such as the government putting a specific time limit on how long the cuts will last for.
Anti-poverty campaigner Sir Bob Geldof told the BBC’s Andrew Marr on Sunday that he feared they could become permanent.
But Health Secretary Matt Hancock told the same programme that it was a “temporary” reduction, which was “entirely reasonable” given that the pandemic had caused a “once-in-300-year economic interruption”.
The government said it will still spend more than £10bn on foreign aid in 2021.
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.