The Tory former PM said the spending cut would damage the UK’s global reputation and make it more difficult to achieve a deal at the Cop26 climate summit in Glasgow this year.
Johnson is refusing to give MPs a vote on his decision to slash aid spending from the legally mandated 0.7% of national income to 0.5%, with Tory rebels believing they have a clear majority to reverse the cut.
The prime minister has also rejected pleas from Commons Speaker Lindsay Hoyle to allow MPs to vote on the decision.
In an emergency debate on Tuesday just two days before the G7 summit of world leaders, Tory rebels criticised the government over the cut and how it has handled the row.
May said the global fund to end modern slavery, an issue key to her prime ministerial legacy, was having its funding cut by 80% as a result of the government’s policy.
She also argued that slashing spending would run counter to Britain’s interests and “have a devastating impact on the poorest in the world and it will damage the UK”.
On the impact on the UK’s world standing, she said: “They (people) listen to us because of what we do, they listen to us because of how we put our values into practice.
“The damage it does to our reputation means that it will be far harder for us as a country to argue for change that we want internationally, that is across the board, including at Cop26 and also including setting out and putting into place the ambitions of the integrated review.
“I only hope that modern slavery is still there on the G7 agenda as it has been in the past.”
What the government is doing is unethical, possibly illegal, and certainly breaks our promise
Former international development secretary Andrew Mitchell, who is leading the rebellion, told MPs the aid spending cut was an “unethical and unlawful betrayal”.
He said: “The way the government is behaving strikes at the heart of our parliament.
“It is precisely because the government fears they would lose that they are not calling one (a vote). That is not democracy.
“I want to argue to the House this afternoon that what the government is doing is unethical, possibly illegal, and certainly breaks our promise.
“It’s not proper and it’s fundamentally un-British and we shouldn’t behave in this way.”
Mitchell also repeated his insistence that trying to win favour in so-called “red wall” working class areas by cutting overseas aid spending was “very patronising” to those voters.
The cut also breaks a pledge to keep the 0.7% target in the 2019 Tory general election manifesto, which helped propel Johnson to an 80-seat parliamentary majority.
“All 650 of us in this House elected at the last election promised to stand by the 0.7%,” Mitchell said.
Responding for the government, Treasury minister Steve Barclay said the cuts were needed given the huge scale of government borrowing to pay for Covid support measures such as the furlough scheme.
He questioned how the rebels proposed raising the £4.3bn required to reverse the cut.
“Leaving the next generation vulnerable to the degree of fiscal threat that would be entailed with a high debt level is not itself morally sound,” Barclay said.
“At the same time, loading ourselves with more debt now might well damage our ability to spend on aid later.”