David Cameron has insisted the UK will enshrine its commitment to foreign aid in law, despite a challenge from his Defence Secretary.
Legislation is due to be tabled in Parliament to make the commitment to increase aid to 0.7% of gross national income by 2013 legally binding.
But in a letter to the Prime Minister, Liam Fox argued that creating a statutory requirement on overseas aid would expose the Government to future legal challenges.
He suggested the principle to commit rather than a specific target should be recognised in legislation, along with an obligation to produce an annual report on whether it was being met.
This was the same approach initially favoured in the case of the military covenant, which Dr Fox was forced to ditch after pressure from Armed Forces charities.
But Mr Cameron pledged that a legally-binding target for overseas aid spending would go ahead.
"The Government is committed to the 0.7% [and] we are going to achieve that in the timeframe that we set out," he said."I think it's the right thing to do because we have a duty to the poorest in our world, even in times of hardship at home.
"I think also, if we take a view of our national self-interest, trying to rebuild some of these broken countries, we'll stop problems being visited back on us at home."
A source close to Dr Fox insisted he was not opposed to the Government's plan to increase international aid, but said: "The issue is simply how best to reflect this in law."
Some Conservative MPs have voiced anger at the decision to increase foreign aid budgets over the coming four years, at a time when spending on domestic priorities - including defence - was being cut back.
Dr Fox's letter, which was obtained by The Times, makes clear that he raised his concerns with the International Development Secretary and the Foreign Secretary before writing to the PM."I have considered the issue carefully, and discussed it with Andrew (Mitchell) and William Hague, but I cannot support the proposal in its current form," wrote Dr Fox.
More stringent monitoring requirements may threaten the Ministry of Defence's ability to report and fund some of its own activities as overseas aid, he added.
And he warned: "I believe that creating a statutory requirement to spend 0.7% carries more risk in terms of potential future legal challenges than, as we have for the covenant, putting into statute recognition of the target and a commitment to an annual report against it.
"The latter would be my preferred way to proceed."
Downing Street said the Prime Minister condemned leaks in all circumstances.A spokesman added: "The aid allocation in the spending review shows our commitment to implementing our pledge to spend 0.7% of gross national income on official development assistance from 2013.
"We are fully committed to enacting the 0.7% commitment into law, in line with the coalition agreement."
Shadow international development secretary Harriet Harman said: "The commitment to increase overseas aid spending to 0.7% of GNI by 2013 was a manifesto commitment by the Tories and the Lib Dems and was repeated in the coalition agreement.
"It must not be the next broken promise.
"Britain's overseas aid saves lives in the developing world, but it is also in our national interest to tackle the underdevelopment which can cause conflict.
"This Tory manifesto promise has been reiterated by the Prime Minister at international forums. He must show that Britain keeps its word.
"The way to show they are not going to break this manifesto commitment is to bring in the promised legislation now. The Government must keep the promise."