The gap in attainment between rich and poor students in academies is widening and is worse than in comparable maintained schools, according to research released today by a Government watchdog.
The revelation is likely to wrench open the debate once more on whether academies are the best means of solving the problems of the English education system.
Education Secretary Michael Gove has based the main pillar of his school reforms on the expansion of the academies programme, arguing that academies, as a group, improve three times faster than maintained schools.
But while the National Audit Office (NAO) report found that the performance of pupils eligible for free school meals (FSM), have English as an additional language or have special educational needs has improved in academies, the gap between them and their peers has actually grown wider in these schools than in maintained schools with comparable intakes.
The report states: "On average, the gap in attainment between more disadvantaged pupils and others has grown wider in academies than in comparable maintained schools.
"This suggests that academies' less disadvantaged pupils benefit from improved standards at the academy more immediately, and that other pupils may take longer to benefit."
While the NAO found that academies have increased the rate of improvement in GCSE results, it said a small number of academies have made little progress and the body warned this performance will not necessarily continue when expanded across many more schools, as planned by the Department for Education (DfE).
Teachers' leaders have greeted the findings as evidence that their criticisms of academies are correct, with the two biggest unions, the NUT and the NASUWT, claiming they give more weight to their argument.
NUT general secretary Christine Blower said: "If ever there was an occasion to say: 'I told you so' this would be it. We have always told this and the last government that taking schools out of local authority control is not the right way to go about schools' improvement."
Chris Keates, NASUWT general secretary, said academies fared no better or worse than maintained schools, adding that academies were only successful in widening social inequality.
"In fact, academy schools risk worsening the outcomes for the most disadvantaged pupils," Ms Keates said.
But David Carter, executive principal overseeing three federated academies in Bristol - Bristol Brunel, Bristol Metropolitan and John Cabot - believes the work he and his leadership teams have done has prevented a pupil's social background from being an obstacle.
"Our GCSE results have shot up this year and I am confident they will continue to improve," Mr Carter said. "I believe passionately in what we do as academies. The parents of the schools we took over had lost all confidence that they could provide a decent education."
The NAO report also found that a significant proportion of academies never received the financial contribution that they were originally promised by their sponsors.
A DfE spokesman said: "The NAO is clear that performance for children on FSM has improved at academies and since 2006 the poorest children have improved at a faster rate at academies than those in other schools.
"The experience of city technology colleges, on which the academy model is based, shows that the attainment gap is much smaller, that FSM pupils achieve more than twice as well at GCSE as pupils on FSM nationally, and that they do better than children from all backgrounds nationally as well. This shows that autonomy works for pupils of all backgrounds."
Sir Michael Wilshaw, principal of Mossbourne Academy
"It's not my experience either at Mossbourne Academy or as director of education for Ark. If you look at any of the schools in the Ark network they are doing as well with kids on free school meals as those who are not. Our greatest claim to success at Mossbourne was that the weakest, most vulnerable do the best.
"I'm sick to death of people hammering academies. They have proved that they have made greater improvements in results than comparable schools."