Boris Johnson has seen off a Conservative rebellion over his plan to cut the UK's foreign aid spending.
MPs voted 333 to 298, a majority of 35, to back reducing overseas aid spending from 0.7% of gross national income to 0.5%.
Ministers have insisted the cut - which will see a reduction of around £4.4bn in assistance - is a "temporary" move amid high government spending caused by the COVID-19 pandemic.
But charities and aid organisations have slammed the result of the vote, with one saying it was a "needless retreat from the world stage" and "akin to cutting the RAF during the Battle of Britain".
Danny Sriskandarajah, Oxfam GB chief executive, said: "The outcome of today's vote is a disaster for the world's poorest people."
Addressing the Commons earlier, the prime minister said the country's public finances are under a "greater strain than ever before in peacetime history".
"Every pound we spend on aid has to be borrowed and, in fact, represents not our money but money that we're taking from future generations," he told MPs.
But the decision has provoked fierce opposition.
A sizable number of Tory MPs have spoken out against the cut, which is written into law and was contained in the Conservative manifesto at the 2019 general election.
A total of 24 Conservatives voted against the government, including former prime minister Theresa May, Jeremy Hunt, Damian Green, Tobias Ellwood, Johnny Mercer, Tom Tugendhat and Andrew Mitchell.
Former prime minister Theresa May said she would rebel against the Conservative whip for the first time, accusing the government of breaking its promise "to the poorest people in the world".
"This isn't about palaces for dictators and vanity projects, it's about what cuts to funding mean - that fewer girls will be educated, more girls and boys will become slaves, more children will go hungry and more of the poorest people in the world will die," she said.
David Cameron, who first enshrined the 0.7% aid commitment in law during his time as PM, said in a tweet that the cut was a "grave mistake".
Another former premier, Sir John Major, said the cut was "the stamp of Little England, not Great Britain".
"The government has blatantly broken its word, and should be ashamed of its decision," he said.
David Davis, a former Brexit secretary, told the debate in the Commons: "I consider myself an economic Thatcherite and yet when I come to choose between money and lives, I always choose lives.
"So, this House should remember that what should be at the forefront of every member's mind today is this is a vote where we are choosing whether or not to intervene to save lives."
Labour leader Sir Keir Starmer said the cut is "damaging Britain's reputation around the world".
"Cutting aid to help the world's poorest during a pandemic is callous - and not in our national interest," he said after the vote.
Deputy Labour leader Angela Rayner said of the government: "Don't let them tell you they want the money spent here in Britain instead, because they voted to leave our hungry kids with no food too."
Liberal Democrat leader Sir Ed Davey said: "It's decisions like this that will make millions of voters, especially in the Blue Wall, move away from the Conservatives in droves."
Ahead of Tuesday's vote, Chancellor Rishi Sunak set out the conditions under which the foreign aid budget will return to the long-held 0.7% promise.
These are when the independent Office for Budget Responsibility judges that the government is not borrowing for day-to-day spending; and that underlying debt is falling.
However, a number of MPs said the conditions laid about by the chancellor would not see aid spending return to 0.7% for some time to come.
As well as the overall cut, MPs also backed this new test in Tuesday's vote.
Andrew Mitchell, a former international development secretary, described the plan as "no compromise at all" and a "fiscal trap for the unwary".
He said the chancellor's conditions had only been met once in the last 20 years.
The new test means aid spending might not return to 0.7% before the next election, which is currently set for 2024.
Existing forecasts, which run to 2025/26, show that in no year is the current budget forecast to be in surplus. Net debt is also not forecast to begin falling until 2024/25.
The PM said "we all believe in the principle that aid can transform lives" and backing the government would "provide certainty for our aid budget and an affordable path back to 0.7% while also allowing for investment in other priorities, including the NHS, schools and the police".
"As soon as circumstances allow and the tests are met, we will return to the target that unites us," Mr Johnson insisted.
This was reiterated by Mr Sunak after the result of the vote was announced, with the chancellor saying: "Whilst not every member felt able to vote for the government's compromise, the substantive matter of whether we remain committed to the 0.7% target - not just now but for decades to come - is clearly a point of significant unity in this House.
"Today's vote has made that commitment more secure for the long-term whilst helping the government to fix the problems with our public finances and continue to deliver for our constituents today."