Theresa May has backed a swelling Tory revolt that aims to force a Government about-turn on controversial cuts to the aid budget.
The former prime minister on Thursday joined a list of 30 Conservative MPs who have vowed to vote for a rebel amendment on the issue in the Commons next Monday.
Opposition parties are also lining up behind the bid to compel ministers to reverse their decision to slash overseas development spending from 0.7 per cent of gross national income to 0.5 per cent.
The move, unveiled by Boris Johnson last year, breaks a Conservative party manifesto commitment and has been heavily criticised.
Tory rebel leader and former cabinet minister Andrew Mitchell expressed "cautious optimism" on Thursday night that he will secure the numbers needed to defeat the Government on the amendment to the Advanced Research and Invention Agency Bill.
Around 40 Conservative rebels are required to overturn the Prime Minister's large majority of 80 seats. The timing of the parliamentary ambush is designed to exert maximum pressure on Mr Johnson ahead of the leaders of wealthy democracies flying into the UK for a major summit in Cornwall next weekend.
One Tory MP urged him to back down ahead of the vote, saying: "Does the PM really want a huge row about Britain breaking its promises on the world stage during the week of the G7?"
Government sources indicated that a climb down would not be forthcoming, however, and questioned whether the rebels' amendment would be rejected, or limited in its scope, because it was tagged on to legislation about the "high risk, high reward" science research funding initiative Aria, rather than a Bill more closely related to overseas development.
The amendment, if selected by Commons Speaker Sir Lindsay Hoyle and approved by a majority of MPs, would force any shortfall of the 0.7 per cent foreign aid spending target to be made up by Aria's proposed £800 million budget.
Ministers conceded last year that it was a "difficult" decision to cut the aid budget, but insisted it was necessary in the wake of the exorbitant cost of the Government's Covid response. The move is designed to be temporary.
Critics in the development sector have warned the reduction will result in tens of thousands of deaths in other parts of the world.
Sir Mark Lowcock, the United Nations' human rights chief, said on Thursday night that it was just "wrong to attempt to balance the books on the backs of the world's starving and suffering".
He said: "The savings are not material in relation to the UK's overall finances."