The conversion of primary schools into academies has not boosted pupils' performance, according to research.
All primary and secondary schools in England were given permission to be run independently of local government in 2010 in an attempt to drive up standards.
But a London School of Economics study has found no evidence to support a benefit to primary students' results during the first two years of "academisation".
The report's authors concluded the Government has "radically restructured its school system under an assumption that academisation delivers benefits to schools and students".
They added: "We consider the first primary schools to become academies in England (between 2010 and 2012) and find no evidence of pupil performance improvements resulting from conversion."
The researchers said one of the reasons an increase was not seen was because schools that converted were already seeing good results.
They used the National Pupil Database to compare the Key Stage 2 results of the 270 primaries that converted into academies between 2010 and 2012 with those that converted much later.
Academies were championed by the then education secretary Michael Gove in 2010 when he wrote to every state primary, secondary and special school inviting them to apply for the status.
In May this year, Nicky Morgan, education secretary at the time, backed down on plans to force all schools in England to become academies.
A Department for Education spokeswoman said: "The academy programme has given good schools the freedom to do what they know works best and helped to turn round underperforming schools that have previously languished under local authority control.
"Results in primary sponsored academies have been improving and we expect that trend to continue."