The prime minister also refused to grant an early vote on the controversy, despite being ordered by the Commons Speaker to allow MPs to have their say.
Tory rebel Andrew Mitchell protested that the cut would lead to a staggering 280 million drugs, tablets and vaccines being “burnt and destroyed” – writing off Britain’s past investment.
“This one act will lead to the maiming, blinding, disruption of lives and deaths of hundreds of thousands of people,” the former International Development Secretary warned.
But Mr Johnson – while noting Mr Mitchell’s “expertise” on the subject – nevertheless insisted he was wrong about £4bn-a-year aid cuts.
Pointing to the aid budget still standing at £10bn, despite the economic emergency caused by the pandemic, he told the MP he did not “accept the characterisation” he had given.
“People of this country should be very proud of what we are achieving,” the Commons was told.
Earlier this month the Speaker Lindsay Hoyle attacked Mr Johnson for refusing to allow the vote promised last year – because, the rebels say, he faces certain defeat.
Mr Mitchell urged the prime minister to “accept and respect” the Speaker’s instruction with a “meaningful vote” before the summer recess, starting in late July.
But, instead, Mr Johnson referred only to a general “estimates” vote – on all government spending – which would not be a specific clash on the aid cuts.
The World Health Organisation warned last week that 280 million lifesaving tablets are likely to expire and have to be incinerated, because UK aid money has been stopped.
It will leave millions of the world’s poorest people at risk from so-called “neglected tropical diseases”, including elephantiasis, trachoma and Guinea Worm.
They are easily preventable but, without treatment, “kill, blind, disfigure and maim”, WHO warned.
It is among numerous bodies agencies alarmed by the impact of slashing aid from 0.7 to 0.5 per cent of national output – breaking a Tory manifesto pledge and, possibly, the law.
The Tory rebels are demanding that the aid cut is reversed from the start of next year, but ministers have hinted it will last for much longer than that.
In April, the foreign secretary Dominic Raab, asked if the cut must be for one year only, to comply with the law – if no fresh legislation is passed – replied: “I don’t think it is quite as straightjacketed as that.”
And he repeated that funding would only be restored “when the fiscal situation allows” – amid huge pressure to hike spending on social care, education and elsewhere.