Continuing to meet a key foreign aid commitment is "much more important" than enshrining it in law, David Cameron said as backbench Tory MPs prepared to renew Commons hostilities that threaten to scupper proposed legislation.
The Government is formally backing a private member's bill tabled by Liberal Democrat ex-Cabinet minister Michael Moore, which would make the aim of spending 0.7% of national income on overseas development a legal requirement.
But Conservative commitment to the legislation - promised in the party's 2010 manifesto and included in the coalition agreement - was undermined by Foreign Secretary Philip Hammond describing it as a "bizarre idea".
Critics , led by Tory MPs Philip Davies and Jacob Rees-Mogg, have tabled a long series of amendments to the legislation, which could see it run out of time for debate today, effectively scuppering its chances of becoming law.
Another Government-backed Lib Dem private member's bill - a bid to ban revenge evictions where landlords force out tenants for complaining about the poor condition of their homes - was scuppered last week when insufficient MPs were present to force it through.
Asked if he would ensure sufficient ministers and MPs were in Westminster to secure the aid bill's passage, Mr Cameron would say only that it would "probably" progress to the statute book.
"We will continue to meet the promise that we made to the poorest people of the world that we would make this aid pledge and meet this aid pledge and we've done that," the Prime Minister said.
"But I think the meeting of it is much more important than the legislation about it.
"It is a private member's bill but nonetheless there will be support for that bill which I'm confident will probably go ahead."
Mr Cameron delivered the lukewarm verdict as he stressed the importance of maintaining the aid spend to boost UK security, at a London conference on the future of Afghanistan.
With experts warning that the Government's latest economic blueprint will mean spending cuts in the UK "on a colossal scale", Mr Cameron has come under mounting pressure from many Tory activists, as well as Ukip, to remove the ring fence protecting aid from cuts.
Dissent over the scale of spending will be further inflamed by the need for the Government to find more cash than expected to meet the target because of a recent recalculation of GDP that pushed the UK's national wealth higher.
The latest figures published in September followed major changes to the Office of National Statistics methodology to bring it into line with international standards, for example including spending on illegal activities such as drugs and prostitution and areas such as research and development for the first time.
Calculations by the Office for Budget Responsibility about the implications for future GDP suggested that would mean £1 billion more in aid over two years in order to meet the target, including £550 million in 2015 spending plans, the Financial Times reported.
The Department for International Development told the newspaper: "We are confident that existing spending plans are consistent with our aid commitments."
Mr Cameron insisted help for Afghanistan and other vulnerable nations was in the UK's direct interest and had already "significantly reduced" the number of threats to domestic security from those places.
In a message to those demanding reduced aid, he said: "I would say to people: we have cut our budget deficit by one half since we came to power. We have kept our commitment to turn around the British economy.
"And the aid payments that we make - keeping a commitment which we made in our election manifesto - is an important part of keeping us safe and secure in the United Kingdom."
A Treasury spokesman said: "T he Autumn Statement confirmed that we will meet the 0.7% aid target in 2014.
"The amount spent on aid in 2014/15 has not been changed. The 2013 Spending Round set budgets for 2015/16, including maintaining ODA funding at 0.7 per cent of GNI in 2015. We are confident that existing spending plans are consistent with our aid commitments."