Former prime minister Theresa May is spearheading a Conservative rebellion against her successor Boris Johnson's plan to slash the UK's foreign aid spending.

The ex-premier accused ministers of breaking their promise "to the poorest people in the world" as she confirmed she would set aside nearly 25 years of Conservative Party loyalty to rebel against the government.

MPs are set to vote on Tuesday on the prime minister's decision - in the face of the UK's huge spend on the COVID crisis - to ditch a promise to spend 0.7% of gross national income (GNI) each year on overseas aid.

After months of disquiet among a group of Conservative MPs over the issue - and the threat of a legal challenge - the government has relented and allowed a House of Commons vote on what it has described as a "temporary" cut from 0.7% to 0.5% of gross national income.

In an attempt to avoid defeat, Chancellor Rishi Sunak has set out the conditions under which the foreign aid budget will return to the long-held 0.7% promise.

These are when the independent Office for Budget Responsibility judges that the government is not borrowing for day-to-day spending; and that underlying debt is falling.

Up to 14 potential Conservative rebels are reported to have sided with the government following the chancellor and Number 10's attempt to reach a compromise.

But Mrs May told MPs she had failed to be convinced by Mr Sunak and Mr Johnson's efforts to see off a rebellion.

"With GNI falling, our funding for aid was falling in any case. To reduce it from 0.7% to 0.5% is a double blow," she told the Commons.

"This is not about palaces for dictators and vanity projects, it's about what cuts to funding mean - that fewer girls will be educated, more girls and boys will become slaves, more children will go hungry and more of the poorest people in the world will die."

And Mrs May claimed Mr Sunak's conditions for returning to the 0.7% commitment were unlikely to be met "for years to come".

The former prime minister mocked the government for focussing on a £4bn cut to foreign aid at a time when it had borrowed 100 times that during the COVID pandemic.

"We're told there will be dire consequences for tax and public spending if this motion is defeated tonight," she said.

"We've borrowed £400bn, where are the dire warnings about that? It seems that £4bn is really bad news but £400bn, who cares?"

Chris Law, the SNP's international development spokesperson, asked those MPs planning to back the government whether they would be "happy" to sign a "death sentence" for those across the world suffering from the coronavirus crisis.

"Are you building forward and leaving no one behind in a global strategy against COVID?," he asked.

"Are you honouring the millions who are losing their lives, and many more millions who will lose their livelihoods, as well as a result of this pandemic?

"Are you happy to sign that death sentence?"

Opening Tuesday's Commons debate, Mr Johnson said the government would "fervently wish" to find the two conditions for returning to the 0.7% commitment had been met.

"I can assure any MP who wishes to make the case for aid that they are - when it comes to me or to anyone in the government - preaching to the converted," he said.

"We shall act on that conviction by returning to 0.7% as soon as two vital tests have been satisfied."

But Labour leader Sir Keir Starmer claimed the government's conditions for returning to the 0.7% commitment were "typically slippery" of the prime minister.

He told MPs that victory for the government meant a cut to 0.5% "will effectively carry on indefinitely".

Earlier, another leading Conservative rebel told Sky News the government's attempt to reach a compromise with MPs was "coming apart at the seams" ahead of the crunch Commons vote.

Andrew Mitchell, the former international development secretary and one of the ringleaders of Tory rebels on foreign aid, described Mr Sunak's plan to avoid a defeat as a "fiscal trap".

"The Treasury proposal, which may have temporarily convinced some of my colleagues, is coming apart at the seams," he told Sky News.

"It's clear that it's a fiscal trap and, if you look back over the last 20 years, there's only one year in which those circumstances would have seen us implement our promise on the 0.7%.

"So, if this vote goes through tonight and we don't win it, it is effectively the end of the 0.7%.

"That has huge effects on the number of avoidable deaths there will be around the world, it has a massive impact on Britain's international reputation.

"And, frankly, it will have quite a strong impact on the Conservative Party who will have been seen to have broken their promise in this very important area and be balancing the books on the backs of the world's poorest people."

Mr Mitchell said he was "confident there will be a substantial rebellion" in the Commons on Tuesday and hoped it would be "enough" to defeat the government and force ministers into a climbdown.

"After all, we're not really the rebels, we're standing by the [Conservative Party] manifesto - it's the government that's rebelling against the manifesto," he added.

"I'm sure people are thinking very carefully about exercising their vote this afternoon."

The former cabinet minister also described it as "shameful" that a group of philanthropists, including the Bill and Melinda Gates Foundation, are set to provide £93.5m emergency funding to cover some UK foreign aid cuts.

"It's shameful that philanthropists have had to divert money from brilliant programmes - for example in combating malaria - to shore up and pass the plate round on the British government's broken promise," he said.

"That's a terrible thing but, also, they are standing up for British taxpayers. Because if the government had got its way and the cuts had gone ahead, all that investment by the British taxpayer will have been wasted.

"And, for example, we would have had to burn or destroy 250 million tablets or vaccines that were meant for those people who were affected by neglected tropical diseases.

"That is the programme that these philanthropists have stepped in to help for just one year only.

"Without them hundreds of thousands, mainly children, would have been maimed or blinded or indeed died as a result."