David Cameron has said the government is to consider spending money from the UK's £10bn aid budget on peacekeeping and other defence-related projects.
Such a move could see funds diverted from the Department for International Development to the Ministry of Defence.
The prime minister explained that security and stability are often needed before development can take place.
Downing Street stressed that the plan would comply with international rules on aid spending.
Fragile statesMr Cameron has been under pressure from his backbenchers to cut the aid budget which he has previously pledged to protect.
He has said he intends to protect all £10bn of Britain's aid budget, promising to spend 0.7% of national income on development.
But, speaking to reporters on his return journey from India, the prime minister said he was very open to the idea of the Department for International Development sharing more of its money with the Ministry of Defence.
Officials said hundreds of millions of pounds could be diverted from aid to peacekeeping and stabilisation operations, particularly in fragile states.
The money would not be used for combat missions or equipment.
BBC deputy political editor James Landale was travelling with the prime minister when he made his comments.
Project decisionsOur correspondent said this shift in priorities would go down well with Conservative MPs and voters who think it was wrong to increase aid while cutting defence.
The question now is what aid projects would be cut to pay for more security operations, he added.
Mr Cameron was returning from leading a British delegation on a three-day trade visit to India.
He became the first serving UK prime minister to pay his respects at the scene of the 1919 Jallianwala Bagh massacre in Amritsar, when British troops opened fire on a public demonstration.
Mr Cameron also held talks with his Indian counterpart Manmohan Singh.
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