Defence Secretary Liam Fox has challenged a plan to enshrine in law the UK's promise to spend 0.7% of its gross national income on overseas aid.
In a letter leaked to the Times, Dr Fox says he "cannot support the proposal in its current form".
A source close to Dr Fox said the issue was not the level of the target but how best to reflect this in law.
Downing Street said it remained fully committed to implementing its pledge in line with the coalition agreement.
The BBC understands Dr Fox's letter was written to Prime Minister David Cameron about five weeks ago.
The defence secretary said that "creating a statutory requirement to spend 0.7%" on overseas aid could lead to legal challenges and limit the government's options on where money was spent.
International aid is one of only a handful of areas, including health spending in England, being ringfenced from spending cuts over the next four years. Most other departments are seeing their budgets slashed - defence spending by 8% by 2015.
BBC Radio 4's chief political correspondent, Norman Smith, said there would be a suspicion Dr Fox was "irked" that more money was going on aid projects when he is having to claw back spending at the Ministry of Defence.
But he added that Dr Fox's concerns, which are likely to anger the prime minister, were shared by other Conservative MPs who believed increasing the aid budget could not be squared with the closure of libraries and children's centres at home.
In the letter, the defence secretary called on Mr Cameron to put the principles, rather than specific commitments, in law following the example set for the military covenant announced on Monday.
Dr Fox wrote: "I have considered the issue carefully, and discussed it with [International Development Secretary] Andrew [Mitchell] and [Foreign Secretary] William Hague, but I cannot support the proposal in its current form.
"In 2009 the proportion of national income spent on ODA [official development assistance] was only 0.52%.
"The bill could limit HMG's [Her Majesty's Government] ability to change its mind about the pace at which it reaches the target in order to direct more resources toward other activities or programmes rather than aid."
A legal target, he added, would result in more "stringent" monitoring of how foreign aid is spent and this may "present risks" to his ability to get funding for certain priority activities from the conflict pool - a fund jointly run by the MoD, Foreign Office and Department for International Development to promote conflict prevention and resolution.
'Issue of trust'
The source close to Dr Fox said: "The defence secretary fully supports the principle of a 0.7% target on international aid. The issue is simply how best to reflect this in law."
Senior Conservative backbencher Patrick Mercer, who is a former infantry officer in the British Army, said he had some "sympathy" for Dr Fox's position - a view known to be shared by many on the right of the party.
"At a time when the armed forces are being cut and cut hard, when they're at war in Afghanistan, Pakistan and indeed in Libya, with other conflicts on the horizon, you can see why the defence secretary is concerned," he said.
But Labour, which along with the Lib Dems also back the commitment, accused Dr Fox of seeking to "mobilise opposition" to it and said legislation was needed to show the government could be "trusted" on the issue.
"There is now a shadow cast over an important manifesto commitment and something which was in the coalition agreement," deputy leader and shadow international development secretary Harriet Harman told the BBC.
"We back what the government said on that. What we have to do is make sure David Cameron insists his cabinet back it. You cannot be in the cabinet and be arguing for a breaking of manifesto promises."
This is not the first time that a private letter from the defence secretary to the prime minister has been leaked to the press.
Ahead of last year's defence review, concerns raised by Dr Fox about proposed cuts in defence spending - which at the time he said were "unsustainable" - entered the public domain. Dr Fox criticised the leak and ordered an inquiry.
The promise to spend 0.7% of national income on aid by 2013 was put in the Conservative manifesto before the last general election and then repeated in the coalition agreement.
A Downing Street spokesman said it was "fully committed to enacting the 0.7% commitment into law, in line with the coalition agreement".
The 0.7% commitment is an international aspiration first mooted by the United Nations in the 1970s and reaffirmed by the world's leading economies in 2002.
Aid and development charity Oxfam said Dr Fox was "wrong" to question the government's approach.
"British aid offers great value to the taxpayer," its chief executive Barbara Stocking said. "For little more than a penny in the tax pound we not only help those in need but also boost our standing in the world and increase our influence in the global corridors of power."
Following a review earlier this year, the government is to stop giving direct development aid to 16 countries - including China and Iraq.
Support for India - the largest recipient of UK aid - is being frozen while ministers have said the overall budget is being more "tightly focused" on those countries most in need such as Pakistan, Bangladesh and Ethiopia.
In future, levels of aid will also be decided in terms of impact and not cost - the most radical shift in policy since the Department for International Development was founded in 1997.