Downing Street insisted today the Government was committed to enshrining its overseas aid spending targets in law, despite objections by Defence Secretary Liam Fox.
No 10 condemned the leak of a private letter from Dr Fox to David Cameron warning that he could not support the plans in their current form.
"Clearly we condemn leaks in all forms," the Prime Minister's official spokesman said, adding that it would be for senior officials to decide whether to mount a leak inquiry.
The spokesman stressed that Dr Fox's concerns would not affect the Government's plan to create a statutory requirement for 0.7% of gross national income (GNI) to be spent on overseas aid.
"It is coalition policy, it is Government policy, it will be made law," the spokesman said.
"There is no question that it is the Government's policy to have the 0.7% of GNI target for international aid. We will introduce a Bill in due course to make that law."
In his letter, obtained by The Times, Dr Fox - who made clear he had discussed the issue with International Development Secretary Andrew Mitchell and Foreign Secretary William Hague - argued that establishing a specific statutory target would open up the Government to future legal challenges.
Instead, he suggested that the target should simply be recognised in legislation, along with a commitment to produce an annual report on whether it was being met.
"I have considered the issue carefully, and discussed it with Andrew and William Hague, but I cannot support the proposal in its current form," he wrote.
A source close to Dr Fox insisted that he was not opposed to the Government's plan to increase spending on aid, saying: "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."
However the leak once again highlighted the uneasy relationship between Mr Cameron and Dr Fox who were rivals for the Tory leadership in 2005.
Last year another private letter from Dr Fox to the Prime Minister - this time warning of the impact of planned defence cuts - was leaked to the press.
Dr Fox strongly denied any responsibility and ordered an inquiry by the Ministry of Defence Police.
The latest leak, however, again chimes with the concerns of many Tory backbenchers who believe that the Government is spending too much on overseas aid at a time of economic austerity.
In contrast, Mr Cameron is strongly committed to the statutory target which he regards as emblematic of the way the Conservative Party has changed under his leadership.
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."