Christian development agencies are urging the Government not to divert aid money to peacekeeping operations.
The plans were unveiled this week by Prime Minister David Cameron who is under pressure to save the Armed Forces from further spending cuts.
However, charities are warning that the move could jeopardise stability in regions benefitting from British aid.
Justin Byworth, Chief Executive of World Vision UK, said: "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."
Christian Aid warned that linking aid to military spending in fragile states could put aid workers on the ground at risk.
"The blurring of the lines between military action and aid delivery could mean that aid workers become associated with those forces, meaning they are not only put at risk, but find it hard to gain the trust of the people they are trying to help," the organisation said.
It further warned that relationships with local partners could be damaged if aid delivery is seen to be attached to a political agenda.
"The best way to bring about peace and stability is through poverty reduction," Christian Aid said.
"We know that conflict is a terrible scourge in our world and disproportionately affects the poorest, and we understand the need for there to be peace and stability in fragile conflict states.
"But life-saving development money should be spent on just that, helping poor communities to build better futures, not on any kind of military intervention."