Conservative MPs mounting a rebellion against aid cuts will gather enough names over the coming days to defeat the government, the Guardian understands, as key figures including Theresa May joined the revolt. Thirty Tory MPs now plan to sign an amendment to the Advanced Research and Invention Agency bill, which has its report stage in the House of Commons on Monday, introducing a new clause reversing the cuts. The amendment is in the name of the former international development secretary Andrew Mitchell and the former Foreign Office adviser and newly elected MP Anthony Mangnall. The rebels will need 40 MPs to defeat the government, but either way ministers will be deeply embarrassed that the vote is happening in the week the UK hosts the G7 summit in Cornwall, where one of the key themes will be help for poorer countries seeking to recover from the pandemic. One source said organisers behind the move had enough assurances from other MPs that gave them confidence they would defeat the government. The MPs say they plan to reveal more names over the weekend to make it clear to the whips that they have the numbers to defeat the government and that the only sensible course is for ministers to make a statement that they will restore the proportion of gross national income spent on overseas aid to 0.7% next year. MPs backing the amendment include May; the former Brexit secretary David Davis; the leader of the One Nation caucus, Damian Green; Tim Loughton, a leading Brexiter; the former defence minister Johnny Mercer; the former Welsh secretary Stephen Crabb; Nus Ghani, a leading trade bill rebel; Bob Seely, a member of the foreign affairs committee; and Ben Everitt, elected in 2019. Mitchell said: “More and more of my colleagues in the House of Commons are supporting this move to stand by our manifesto promise. With our economy returning to growth, there is no justification for balancing the books on the backs of the world’s poor. “With G7 leaders coming to Britain next week, there is an opportunity for us to reclaim our rightful place on the global stage. Britain’s national interest is not being served by the devastating impact these cuts are already having on the ground and the unnecessary loss of hundreds of thousands of innocent lives. We urge the government to think again.” Ministers cut the proportion to 0.5% citing the UK’s dire economic outlook due to Covid. The government has said it will restore the aid back to 0.7% when circumstances allow but has set no metrics or timings for this to happen, leading some observers to believe Boris Johnson initially saw the cut as a permanent move that would be popular with red wall voters. The government has found itself under increasing pressure as the impact of the cuts on government programmes starts to drip through in high-profile areas such as Yemen, Syria and the Democratic Republic of the Congo. Some priority areas for the government such as girls’ education have been cut, leaving ministers looking foolish in promoting such causes at international summits. Green, the former cabinet minister and chair of the One Nation Conservative group, said many colleagues across the ideological divide simply did not agree with breaking a manifesto commitment. “This was a clear and brave move by the Conservative party to meet this commitment and many of us think it’s not only bad in and of itself for a lot of hugely disadvantaged people across the world, but it’s also the wrong place to look for cuts particularly at this time,” he told the Guardian. “Being global Britain comes with responsibility, including being a global player in aid.” Loughton, a key member of the party’s pro-Brexit faction, said he would join the rebels, calling it a “serious breach of faith” to ditch the commitment that was in the manifesto 18 months ago. “I was tremendously proud that it was a Conservative-led government which enshrined the 0.7% commitment into law and gave a lead to the world by showing we were prepared to put our money where our mouth was,” he said. Neil Parish, the Tory MP who chairs the environment, food and rural affairs committee, said: “A failure on aid is a failure on climate. Many of my colleagues are simply waking up to this and that’s why they’re prepared to sign on to this amendment. “World leaders are about to arrive in Cornwall for the G7, and Cop26 is just a few months away, but instead of displaying all we’ve achieved when it comes to climate and global health, we’re undermining our own work and disregarding a core manifesto commitment.” Charity leaders praised the efforts of MPs to stop the cuts, a move which is expected to be backed by Labour and other parties. The chief executive of WaterAid, Tim Wainwright, said: “It’s just a few days before world leaders arrive in Cornwall for the G7 and the UK government is currently providing a depressing case study in how not to respond to the Covid-19 pandemic and climate crisis. No wonder even its own MPs have had enough of this unnecessary and unconscionable act of sabotage.”