Charities have rounded on David Cameron after he suggested aid cash could be spent by the military.
The "morally wrong" proposals could put aid workers in danger, they claimed, urging the government not to "pilfer" from the international development budget.
Cameron has said he is "very open" to the idea of pooling resources between international aid and the cash-strapped Ministry of Defence.
It could lead to hundreds of millions of pounds from the Department for International Development (DfID) being spent on peacekeeping and other non-combat activities.
DfID said it was already playing a "key role" in warzones.
Cameron's suggestion will be welcomed by some Tory backbenchers, who are fuming that the aid budget is protected amid sweeping cuts to defence.
But it has provoked an outcry from campaigners who said it threatened the British government's hard-won reputation in international aid.
Justin Byworth, chief executive of World Vision UK, said: "The British Government leads the way on good aid spending - and diverting more money to peacekeeping operations could put that in jeopardy.
"The Prime Minister's correct that security and peace are essential, but long-term stability is about far more than peacekeeping. In reality, it's a complex mix which includes nutrition, health, education and protecting vulnerable children, which all help to prevent conflict and empower people to cope with challenging circumstances.
"Spending on peacekeeping from the aid pot is already limited by international standards. We'd be concerned if this statement means these standards are being challenged."
Sorcha O'Callaghan, head of humanitarian policy at the British Red Cross, said: "Humanitarian agencies working in conflict need to be able to distance themselves from military objectives in order to be seen to be neutral.
"Blurring the lines between aid and military objectives will not only reduce the resources available to meet the needs of the most vulnerable, it also risks the access and safety of aid workers."
ActionAid spokesperson Melanie Ward said defence needs should have “no bearing on how aid is spent”.
She said: “The purpose of UK aid is to reduce poverty and this is enshrined in law. To pilfer from aid to pay for defence would be morally wrong and practically misplaced.
“Of course, when UK forces are in action they should always coordinate closely with international development experts to make sure that the needs of local people are met, but a raid on aid would provide no solution. Instead it could well deprive some of the world’s poorest people of life-saving support and undermine UK efforts to promote human security globally.
“Military objectives and defence needs must have no bearing on how aid is spent, particularly as it has already been shown by US army-led activities in Afghanistan that use of the military in development projects can put lives at risk.”
A DFID spokeswoman said: “All DFID’s aid spend is in line with internationally agreed guidelines.
“As part of the Strategic Defence and Security Review, we pledged to invest 30% of our bilateral aid in fragile and conflict-affected countries by 14/15 and we work closely with the Foreign and Commonwealth Office and the Ministry of Defence to make sure our approach is joined up, including through the Conflict Pool which provides funding for conflict prevention.”