Boris Johnson faces being defeated by a Conservative Party rebellion with 30 Tory MPs, including former prime minister Theresa May, having signed an amendment to reverse savage cuts to Britain’s international aid budget.

The number of rebels doubled overnight with May joining one-time ministers Stephen Crabb and Johnny Mercer in adding her name to a rebel Tory amendment aiming to restore British foreign aid spending to 0.7% of national income.

On Wednesday, 15 Conservative MPs, including the chairs of the defence and foreign affairs select committees, Tobias Ellwood and Tom Tugendhat, backed an amendment to increase foreign aid spending from the current rate of 0.5% announced by foreign secretary Dominic Raab last year.

A source told openDemocracy that the government could be set to reverse the cuts rather than risk the prospect of a Commons defeat on Monday.

“The rebels are pretty confident they have the votes,” the source said.

The scale of the cuts, revealed by openDemocracy earlier this year, has seen spending in some of the poorest countries cut by as much as 88%.

The rebel amendment was tabled by former international development secretary Andrew Mitchell and 2019 intake Tory MP and former foreign office adviser Anthony Mangnall.

The rebels – who are said to be confident they have the numbers to defeat the government – are looking to force a Commons defeat on Johnson on Monday, just days before the start of the G7 summit in Cornwall.

The group of rebels includes the leader of the One Nation Caucus, Damian Green; leading Brexiteer Tim Loughton; former defence minister Johnny Mercer; former Welsh secretary Stephen Crabb; leading Trade Bill rebel Nus Ghani; foreign affairs committee member Bob Seely; and 2019 intake MP Ben Everitt.

The British government has slashed spending on foreign aid by more than £4bn. UK aid to Yemen, one of the world’s most war-torn states, was reduced from £197m pledged in 2020 to £87m this year and humanitarian funding for Syria has been halved.

In the wake of openDemocracy’s revelation, Boris Johnson was accused of a “grotesque betrayal” and of failing “some of the most hungry, terrified, hurt people in the world.”

The cuts include huge reductions in research funding and as much as 80% cuts to British global anti-corruption work.

Speaking Thursday, Andrew 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.”

Mitchell has tabled an amendment to the Advanced Research and Invention Agency (ARIA) Bill, a piece of legislation which establishes a new “high-risk, high-reward” research agency backed with £800m of taxpayers’ cash to explore new ideas – and which, controversially, will be exempt from FOI.

The amendment, if selected by the speaker, would introduce a new clause to the ARIA Bill, which has its report stage in the House of Commons on Monday. It would be a technical change to enforce the 2015 International Development (Official Development Assistance Target) Act, which obliges the government to meet the 0.7% target in 2022.

The amendment, which has wide cross-party support, is backed by every former chair of the public accounts committee, including Tory MPs David Davis and Edward Leigh and Labour’s Margaret Hodge and Meg Hillier, its current chair as well as Sarah Champion, chair of the international development committee and Labour shadow development and foreign secretaries Preet Kaur Gill and Lisa Nandy.

One of the Conservative rebels, Caroline Nokes, said the group had put “an awful lot of work” into designing their amendment, and she was “very hopeful” it would be selected.

But, in a BBC News interview, she added the group was “entirely in the hands of the speaker at this point”.