The government is being urged to take stock of its academies programme before building any more of the state-funded independent schools.
Gordon Brown has stressed his support for academies
A report for the Trades Union Congress argues that their impact on achievement is not clear enough to justify the expansion that is planned.
It wants an independent review of their effectiveness before plans to boost the number from almost 50 to 400 go ahead.
Ministers have insisted academies are successfully tackling underachievement.
Prime Minister Gordon Brown has expressly backed the academies programme and says it is set to continue.
Tony Blair, before stepping down as leader, suggested a government target to create 200 academies by 2010 should double to 400, albeit without a date.
One day all schools should be academies or trust schools, he said.
But teaching unions are not convinced of their merits and complain that they are not automatically recognised as they are in mainstream state schools.
The TUC report looks at how academies have performed against the objectives set out for them by former education secretary David Blunkett.
It acknowledges that it is difficult to judge academies' success in some areas, such as raising educational achievement, against schools of similar characteristics.
It claims there is a mixed picture with some doing better than other inner city schools, targeted for extra help, and some doing worse.
Review of sponsors
This, it argues, is part of the reason it is calling for the review.
The report also suggests there is little collaboration between academies and neighbouring secondary schools, which was one of Mr Blunkett's original objectives.
Academies have not opened up their facilities for local use in many cases, as had been planned, because tax regulations would then have made them liable for VAT on the full construction costs of the schools, the report says.
This is something Gordon Brown changed in his last Budget.
It is also difficult to judge whether academies had been effective in reducing absences and decreasing exclusions, the report adds.
It concludes there is no longer enough clarity about the government's overall strategy for improving secondary schools - especially about how the academies programme sits beside other initiatives.
It calls for an independent panel of academics and education policy experts to assess how well government's plans to improve the education of teenagers is working.
It also wants to see a review of the kinds of organisations that can sponsor academies - with local authorities and public sector bodies being given greater encouragement, and with academies made more accountable to staff, parents and local communities.
TUC general secretary Brendan Barber said the recent announcement by new Children's Secretary Ed Balls that local authorities would have a greater say in the planning of future academy schools was a step towards regaining full control of them.
But he added: "Unless a thorough review is carried out, academies will continue to grab the headlines while denying recognition to other initiatives that quietly get on with the business of improving the educational prospects of many of the UK's disadvantaged teenagers."
Schools Minister Andrew Adonis said: "The evidence shows that academies are successfully tackling both failure and under-performance.
"Academies are improving results at a much faster rate than the national average.
"An independent study by the National Audit Office this year concluded that academies were on course to deliver good value for money and were raising attainment rapidly in deprived areas."