The charity's speaking after the Independent Commission for Aid Impact (ICAI), which scrutinises UK aid spending, published a report which found the Prosperity Fund needs to improve how it delivers aid.
ICAI found the £1.3 billion Fund has not yet delivered any major programmes to alleviate poverty - one of its main aims as well as to create opportunities for international businesses.
The watchdog recommended the Prosperity Fund improves its transparency, with a lack of information about what it does available to the public.The watchdog recommended the Prosperity Fund improves its transparency, with a lack of information about what it does available to the public.
The fund is set to run until 2021.
The Prosperity Fund is currently being overseen by the National Security Office, however Christian Aid argues expertise in the Department for International Development is being wasted and they would administer the money much more effectively.
Christian Aid's Head of Private Sector Kenneth Boyce said: "This report shows in the starkest terms why it is vital that there is one department in government 100% committed to poverty reduction.
"Devolving the aid budget to poorly managed cross-government funds leads to conflicts of interest, lack of rigour, and loses the focus on the world's poorest people.
"For the public to trust the UK's aid budget, they need to know that the money is helping those in need. That requires full transparency and clear criteria for effectiveness. Today's report shows that so far the Prosperity Fund is failing both these tests and is currently not fit for purpose."
A UK government spokesman said: "Sustained economic growth is the only long term solution to poverty and the Prosperity Fund supports the vital economic development needed to help middle-income countries – where more than 60% of the world's poorest live - to stand on their own two feet and become our trading partners of the future.
"Overseas Development Assistance spend under the Fund is and will continue to be fully consistent with UK law under the International Development Act and OECD DAC criteria.
"As ICAI acknowledge the Fund has made significant progress in a short time frame and we are already implementing the vast majority of ICAI's early recommendations, including on transparency."
