Academies represent a "high-risk" investment for wealthy donors who want to support education, according to a report by a think- tank which advises on giving to charities.
The report, by New Philanthropy Capital, said there was not enough evidence to say whether the privately-sponsored state schools are the best way for donors to spend their money. Mixed Ofsted reports and exam results raised serious doubts over the "cost-effectiveness" of the scheme.
The report concluded: "Academies are a risky investment: they can and do fail."
The Prime Minister aims to use academies to transform state education in England's inner cities. The study follows the "cash for honours" row over claims that business figures were told they could get knighthoods or peerages in return for backing academies.
The think-tank, which advises philanthropists on how to give money more effectively, said donors would be better spending the pound;2 million required for academy sponsorship on other educational causes.
"Academies show mixed results for their pupils," the report said. "While the proportion achieving five grade A* to C at GCSE compared to their predecessor schools is greater, some have been heavily criticised by Ofsted for failing to provide a satisfactory standard of education.
"Sponsorship of an academy is high risk, and success, if it comes, will not be immediate."
Many academies have been opened in new buildings designed by leading architects such as Lord Foster and Lord Rogers. The report said that the pound;25m price for setting up an academy in brand new buildings looked "very expensive".
"State-of-the-art buildings look good and pupils and teachers are very proud of them, but there is scant evidence of the link between this capital investment and pupil attainment. Undoubtedly, the case for academies would be stronger if they did not cost so much to build."
The report added: "Perhaps the most powerful criticism of academies is the pound;8m difference between the cost of building an academy and the cost of building a conventional school."
The charity's report suggests a range of possibilities, including supporting anti-bullying groups and charities helping children with special educational needs.
In exchange for up to pound;2m, academy sponsors are given control of a number of key areas, including deciding the school's ethos and appointing governors. The Government pays the rest of the bill - typically pound;25m.
Ministers want to create 200 academies by 2010.
A spokesman for the Department for Education and Skills insisted that academies did not represent poor value for money.
"Support from sponsors is buoyant and we are already halfway to our target of having 200 academies open or in the pipeline by 2010," he said.
The pound;8 million "funding gap" was wrong, the spokesman added.