Hundreds of millions of pounds will be diverted from Britain's growing aid budget to ease the impact of defence cuts on the Armed Forces.
David Cameron has signalled that money from the Department for International Development will be used to help pay for "stabilisation" operations by the Armed Forces in fragile states in Africa, the Middle East and Asia.
Providing basic security and peacekeeping is an important part of development, the Prime Minister said.
The shift from Mr Cameron comes after defence chiefs raised fears that the Treasury is planning more cuts in the Forces' budget. Government sources said money from the aid budget will help offset some of the cuts, but not all.
Taking money from DFID is likely to please Conservative MPs who are unhappy that aid spending is rising sharply even as other budgets shrink.
The Ministry of Defence budget is being cut by 8 per cent over four years, leading to the loss of 30,000 Armed Forces posts. By contrast, DFID's budget is growing by a third as Britain tries to hit a United Nations target of spending 0.7 per cent of its domestic product on overseas aid.
Speaking in India, Mr Cameron said that the aid and defence ministries already worked very closely together.
He added: "If you're asking, can they work even more closely together, and make sure the funds at our disposal are used to provide basic levels of security and stability in deeply broken and fragile states then, yes, I think we should – that is an important part of development."
DFID, the MoD and the Foreign Office already pay into a central "conflict pool" that funds some stabilisation work. Mr Cameron said that the principle of sharing money could be extended. He said: "Can we do more? Can we build on that approach? I am very open to ideas like that."
The pledge to increase aid spending was a central part of Mr Cameron's attempt to portray the Conservatives as compassionate and generous. Some Tories think it politically unsustainable in an age of austerity.
The pressure on the Armed Forces from new commitments in north Africa has made additional defence cuts more controversial. Mr Cameron said: "We have our moral responsibilities for tackling poverty, and we also have our national security responsibilities for mending conflict states. We should see DFID in that context."
Exactly which items of state spending count towards the 0.7 per cent target for aid is decided by the Organisation for Economic Cooperation and Development, a club of industrialised nations.
Its guidelines rule out using aid money for combat operations. But the rules are less clear on peacekeeping missions.