The new Government must ensure that academies' exam results are open to full public scrutiny, a think-tank said this week.
Research by Civitas shows that some academies are substituting academic subjects for "highly questionable" "pseudo-vocational" courses to produce strong headline GCSE league table figures.
But Anastasia de Waal, Civitas deputy director, was unable to obtain detailed qualification figures from most academies because the Government does not collect the data. And, unlike other state-funded schools, they are not covered by the Freedom of Information (FoI) Act.
In March, then justice minister Michael Wills said the FoI legislation would be extended to cover academies from October 2011. But nothing was done before the general election, and Ms de Waal is concerned that the change could be lost under the new coalition Government.
She argues that with the Conservatives and Labour both committed to opening hundreds more academies, ensuring that they are transparent and accountable is more important than ever.
"It is completely crucial that academies are brought under the FoI (Act)," Ms de Waal said.
"We are talking about a new form of school which has pretty much become the flagship model because it is seen to turn around schools in terms of GCSE results. But we don't know what is actually happening in terms of those results."
The headline league table benchmark of the percentage of pupils achieving five A*-C GCSEs or equivalent, including English and maths, allowed Labour and the Tories to claim that academies' results were improving at twice the national rate.
But Civitas found it was impossible to obtain the detailed academy results behind this benchmark.
The think-tank surveyed the heads of 118 academies and found that although 80 per cent saw exam results as an indicator of their school's progress, only 55 per cent believed they should be made publicly available, broken down by subject.
Even fewer heads - 43 per cent - agreed to release their results to the think-tank.
Where detailed breakdowns were obtained, they revealed that a high proportion of their GCSE success was achieved through what Ms de Waal calls "pseudo-vocational" qualifications instead of traditional academic GCSEs.
One London academy said it did not think it should have to release its results "because it will identify the subjects that the academy has chosen, through its freedoms, not to prioritise, e.g. separate sciences, geography etc ...".
Ms de Waal believes the current situation would be particularly contradictory under a part-Conservative administration because the party has taken a strong stand against schools teaching "soft" subjects.
"The very fact that so many academy principals said academies shouldn't be subject to FoI is clearly a worry," she said.
"There needs to be full transparency. Why should academies be treated any differently than other schools?
"We have these problems of equivalence because schools are under such enormous pressure to ratchet up results and they are turning to spurious vocational subjects for pupils who are already among the most deprived."
A Ministry of Justice spokesperson said: "We are in the very early stages of a new government and ministers will wish to consider the full range of justice policy."