Theresa May has called on Boris Johnson to restore the UK’s commitment to spend 0.7 per cent of national income on international aid, warning him that his cuts “will have a devastating impact on the poorest in the world and damage the UK”.

The former prime minister was one of a string of senior Conservatives who lined up to savage Mr Johnson’s decision to cut aid spending to 0.5 per cent of gross national income to claw back some of the borrowing resulting from the Covid crisis.

In an emergency Commons debate taking place just hours after Downing Street confirmed that Mr Johnson will defy a demand from Speaker Sir Lindsay Hoyle to give MPs a binding vote on the issue, Ms May said that the £4bn cut breached a Conservative manifesto commitment.

And she said it harmed the UK’s standing in the world just days before the G7 group in Cornwall at which Mr Johnson will be the only world leader who is cutting aid in response to the coronavirus pandemic.

Ms May said that Mr Johnson’s decision meant an 80 per cent cut in the budget of the global fund to end modern slavery, putting children at risk of sexual exploitation.

Organisations like this were being forced “to go cap-in-hand to other governments to make up for the shortfall caused by the UK’s decision to cut international development spending”, she said.

The damage to the UK’s reputation will “make it far harder for us as a country to argue for the change that we want internationally”, either at the COP26 climate change summit which Mr Johnson chairs in November or in the PM’s ambition to create a “global Britain”, warned Mrs May.

And she told MPs: “This cut from 0.7 will have a devastating impact on the poorest in the world ant it will damage the UK.

“I urge the govenrment to reinstate the 0.7 per cent. It is what it promised, it will show that we act according to our values and it will save lives.”

The emergency debate was secured by former international development secretary Andrew Mitchell, after parliamentary rules blocked a rebel attempt to force the government to reverse the cut on Monday.

Mr Mitchell said that, despite his 80-strong majority in parliament, Mr Johnson would face defeat by a margin of up to 20 in any parliamentary vote on aid, with opponents including 16 former ministers, eight Commons committee chairs and 12 Privy Councillors, backed by every living prime minister and every former leader of a major party.

He rejected the description of those demanding a return to the 0.7 commitment – enshrined in law by David Cameron – as rebels, telling MPs: “It is the government that is rebelling against a clear and indisputable commitment.”

Mr Mitchell accused the prime minister of undertaking the “unethical and unlawful betrayal” not because of cash constraints following Covid, but in order to pander to what he believed were the opinions of voters in so-called Red Wall seats in the Midlands and North snatched from Labour at the last election.

UK funding for the PM’s supposed development priority of girls’ education had been cut by 25 per cent as a result of breaking the commitment, while 10m people had lost UK support for clean water provision and 250,000 people at risk of starvation were missing out on UK food aid each week, he said.

With France embracing the 0.7 per cent United Nations target, Germany going beyond it and the US seeking a $14bn increase in aid spending, “we are the only ones going backwards” at the G7 summit, said Mr Mitchell.

Comparing the PM’s refusal to put the cut to a vote in parliament to the actions of the monarch whose high-handed actions provoked the English Civil War, the former cabinet minister said: “The executive is accountable to parliament… and that applies in all circumstances, whether the executive is being run by King Charles I or Boris Johnson.”

Shadow international development secretary Preet Kaur Gill told the Commons: “We clearly have a government in hiding, a government that has tried over and over again to avoid scrutiny and accountability for the cuts that they have imposed.

“We are no strangers to hyperbole in this House, but it really is no exaggeration to say the cuts to the aid budget by this Government has cost people their lives.

“It is utterly shameful.”