Gordon Brown's 'Marshall Plan' for Africa will hugely reduce aid over the long term, according to the World Development Movement.
The Treasury claims the scheme, which involves borrowing money on international bond markets, will help meet the Millennium Development Goals by increasing aid by $50bn a year until 2015.
But a WDM study claims aid will fall after 2017, as the money will have to be repaid.
"The worst-case scenario is that the International Finance Facility steals from future aid budgets and reduces the total amount of aid over the long term, while imposing the same failed policies on poor countries," said WDM head of policy, Peter Hardstaff.
WDM says schemes to increase aid, such as a tax on air fuel, deserve a higher priority.