Baroness Sugg expressed sadness that she was leaving a job she loved, but said it was “fundamentally wrong” to abandon a commitment to spend 0.7 per cent of gross national income on development.
“This promise should be kept in the tough times as well as the good," she told Boris Johnson, adding that the UK’s leadership on the issue had saved and changed millions of lives.
Former Conservative prime minister David Cameron described the announcement as a "very sad moment" for Britain.
The Conservative manifesto at last year’s general election committed the government to spend 0.7 per cent of national income on overseas aid.
But unveiling his spending review Mr Sunak told MPs that the pledge was “difficult to justify” to the public alongside the highest peacetime levels of borrowing on record.
Instead, ministers will spend 0.5 per cent in 2021, around £10bn.
But the government faces a potential Tory rebellion when legislation for the move comes before parliament.
Conservative MP and former international development secretary Andrew Mitchell warned the government it would be "the cause of 100,000 preventable deaths, mainly among children".
A former Foreign Office minister Harriet Baldwin told MPs she felt "ashamed" her party was implementing the cut.
Ministers are set to stage a fightback on Thursday as they outline plans to “do more with less” when it comes to the foreign aid budget.
It is understood the foreign secretary Dominic Raab will in future personally sign off all overseas development spending, in a bid to ensure money is spent more effectively than it has been at some points in the past.
The move has attracted condemnation from church leaders, charities and senior members of the armed forces, as well as Conservative politicians.
The Archbishop of Canterbury Justin Welby said the decision was "shameful and wrong".
The former head of the army Lord Dannatt warned the cut would "significantly reduce our soft power and thereby reduce the impact of the so-called global Britain''.
Nobel Prize laureate Malala Yousafzai said she was “deeply disappointed” at the cut and urged ministers to think again.
First minister of Scotland Nicola Sturgeon described the cut as "a political gesture to the right wing of the Tory party".
The decision, which had been expected, has previously been criticised by other former prime ministers Sir John Major and Tony Blair.
Mr Sunak did not make any promise of when the 0.7 per cent figure would be met, saying only "when the fiscal situation allows".