Boris Johnson was struggling to contain a Conservative revolt as a Foreign Office minister resigned after the chancellor, Rishi Sunak, announced he was breaking a manifesto promise by cutting the overseas aid budget by a third and ending the Tory commitment to spend 0.7% of gross national income on aid. Lady Sugg, whose brief includes sustainable development, submitted her resignation to Johnson in protest against the cut. Sunak insisted the cut reflected people’s priorities at a time of unprecedented economic emergency. But Sugg, a close ally of David Cameron and enthusiastic advocate of girls’ education, did not appear at the Lords dispatch box to answer questions on the aid cut that formed the most controversial measure in Sunak’s spending review. In her resignation letter, she wrote: “Many in our country face severe challenges as a result of the pandemic and I know the government must make very difficult choices in response. But I believe it is fundamentally wrong to abandon our commitment to spend 0.7% of gross national income on development. “This promise should be kept in the tough times as well as the good. Given the link between our development spend and the health of our economy, the economic downturn has already led to significant cuts this year and I do not believe we should reduce our support further at a time of unprecedented global crises.” Speaking in the Commons, Sunak said: “Sticking rigidly to spending 0.7% of GDP on overseas aid is difficult to justify to the British people,” adding that aid spending would fall to £10bn in in 2020-21. The aid target would be cut to 0.5%, Sunak confirmed, adding he hoped the 0.7% target could be restored when the UK’s finances allowed it. A number of senior MPs warned Sunak he was endangering the government’s leadership role at a critical time for the world, including the British hosting of the UN climate change conference next year. Andrew Mitchell, a former Conservative international development secretary, said the aid cuts “will be the cause of 100,000 preventable deaths, mainly among children. This is a choice I for one am not prepared to make. None of us will be able to look our children in the eye and claim we did not know what we were voting for.” The former foreign secretary Jeremy Hunt said: “To cut our aid budget by a third in a year when millions more will fall into extreme poverty will make not just them poorer but us poorer in the eyes of the world.” He added: “People will worry we are abandoning noble ideas that we have done more to champion than anyone else.” Tobias Ellwood, the Tory chair of the defence select committee, condemned the move, saying the UK cannot genuinely claim to be global Britain “when our hard power is not matched by soft power”. Peter Bottomley, the Tory father of the house, said he would work across party lines to block the cut. Pauline Latham, the Conservative MP for Mid Derbyshire, said the aid budget would be decimated, “leading to more child marriages, more instances of early childbirth FGM, more domestic violence, and we will not be vaccinating millions”. The cut in aid spending contrasted with a large three-year increase for the defence budget, but Sunak clearly decided to brush aside warnings that Britain’s commitment to foreign aid symbolised an outward looking and generous UK. Setting aid spending at 0.7% was first adopted in principle as a target by the UK in 1974 and enshrined in law in 2015, two years after the target had been met for the first time. In practice the law only requires the foreign secretary to provide a report to parliament explaining why the target has not been met. The £15bn aid budget in this financial year is already cut by £2.9bn due to the sharp decline in growth, but was expected to recover next year along with the projected 5.5% growth in the British economy. Hopes that the target would be cut to 0.5% only temporarily in a measure that might not require primary legislation were dashed, as were suggestions that more of the aid budget could be redefined and given to other departments, including assistance with security and climate change, two preconditions for development. However, the requirement to introduce primary legislation will give an opportunity for the reformed Tory One Nation group to show if it can test the government’s overall 80-seat majority. A fortnight ago the group published a pamphlet authored by the former foreign secretaries Hunt and Malcolm Rifkind, and Mitchell. Many Tory MPs also warned Johnson’s calling card on the world stage would be thrown away ahead of the UK’s chairmanship of the G7 this year and it hosting the UN climate change summit in Glasgow. Sunak also resisted opposition from more than 180 aid groups, two former prime ministers – Tony Blair and David Cameron – as well as the archbishop of Canterbury. There were also hints of opposition from US Democrats, although the US itself has never taken on the target. The abolition of the target has been long sought by Conservative rightwingers, who say it plays well with swing voters at a time of domestic austerity. Many Tory MPs claim the figure is arbitrary and leads to a rush in inefficient spending at the end of the financial year. The government’s aid watchdog, the Independent Commission for Aid Impact, rejected this argument in a report published this week. The 0.7% figure was adopted internationally as a target by the UN general assembly in October 1970 following a World Bank report recommendation. In 2005 the 15 countries that were members of the EU by 2004 agreed to reach the target by 2015, but few in practice did so. The target was also the reference point for the UK when it chaired the G8 Gleneagles summit in 2005, which was seen as the high-water mark of international efforts to restructure unsustainable debt in the world’s poorest countries. Sarah Champion MP, chair of the international aid select committee, said the cut in aid was devastating. She said: “Within six months a specialist development government department has been scrapped, this year’s budget has already been slashed and next year’s too by even more. I think we can wave goodbye to the development superpower status that we have proudly had for so long.”