Conservative enemies of development spending could unite to block a bid to force future governments to spend 0.7% of gross national income (GNI) on aid.
Their possible intervention emerged after former Liberal Democrat Cabinet minister Michael Moore tabled a private member's bill proposing to legislate on the issue.
All three of Westminster's mainstream parties supported such a move in their 2010 general election manifestos and will not stand in the way of the bill as it progresses through the Commons.
But private member's bills are vulnerable to a form of filibustering by MPs opposed to them – creating the possibility that a small number of opponents could prevent Moore's bill from becoming law.
The bill would create a legal requirement that the UK government meets the UN's target of contributing 0.7% of GNI – roughly £12 billion each year – as official development assistance.
The Treasury has already achieved the target after foreign aid spending received a 28% bump last year.
Now Moore wants to make it permanent – in a move which could trigger a rebellion from some Tory MPs.
"UK aid is making a huge contribution to people across the world, but at a time when the global economy remains weak and the gap between rich and poor remains appallingly wide, the need for a strong British commitment to development aid is more important than ever," Moore wrote in an article for the Lib Dem Voice blog.
The Conservative party is offering lukewarm support for the legislation, despite the pledge being included in the 2010 coalition agreement.
Government ministers will speak in favour of the bill but backbench MPs will not be prevented from attempting to 'talk out' the legislation.
One MP who has already declared war on Moore's bill is Peter Bone, who is concerned a set target will result in the money being spent inefficiently.
"I can quite understand why a Liberal Democrat would want to go ahead with that because they don't believe in financial management and are keen to get rid of taxpayers' money," he said.
"But the issue goes back to how does a percentage of GNI ever relate to need?
"We need to have overseas aid based on need.
"What happens [with a fixed target] is you get close to the end of the financial year and lots of money gets doled out by the overseas aid department because they have to reach the target."
The international development (official development assistance target) bill receives its first reading in the Commons today. It will be debated on the floor of the Commons in September.