One in 10 secondary schools is now an academy
More than one-in-10 state secondary schools in England are now academies, outside of local authority control.
The government says there are now 407 primary and secondary schools with this state-funded independent status.
The Education Secretary Michael Gove said the change amounted to the "fastest rate of education reform in English history".
The National Union of Teachers said it was creating a "patchwork of unaccountable provision".
The ATL teachers' union claimed the increase in academies would prevent "sensible planning" and would open up schools to profit-seeking private firms.
The figures from the Department for Education show that at the beginning of the new term there are 371 secondary schools with academy status, representing 11% of the total.
It means that the number of academies has doubled since last year's general election.
'Free from interference'
Academies have greater control over their budgets and can set their own pay and conditions for staff.
There is also an expectation for successful academy schools to set up partnerships with weaker schools.
This latest milestone for the academy programme marks the point at which a majority of academies will have been created by the current coalition government.
There were 203 academies at the time of the general election last year - with those created under Labour bringing in an external sponsor and often replacing failing schools.
A further 68 of these sponsored academies, tackling under achievement, have since been opened.
Mr Gove says the programme will drive up standards.
"Head teachers and teachers - not politicians and bureaucrats - know best how to run schools," he said.
He has promised to accelerate the academy programme - offering a fast-track for high-achieving schools and inviting all schools to apply for academy status.
The government has argued that giving schools greater autonomy will help to improve standards.
"Schools are taking up our offer to become academies because they recognise the huge benefits of being an academy - more autonomy, more power to teachers, and an opportunity to thrive, free from interference from government," said Mr Gove.
"We have only been in power for seven months and the act allowing schools to become academies was only passed in July," he told BBC News.
"It's the fastest rate of education reform in English history."
So far 136 schools have converted to academy status - with a further 254 applications in the pipeline.
NUT general secretary Christine Blower said the figures showed schools were "thinking twice" about taking up academy status.
"What we need to see for the benefit of all our children's future is a democratically-accountable education system operating within the local authority," she said.
Chris Keates, general secretary of the NASUWT, said schools which converted to academies were being motivated by extra funding.
"The clear motivation for academy status is that most schools are being duped into believing that they will get extra money at a time when schools and education are facing savage cuts."
The ATL teachers' union attacked the expansion of academies.
"The actions of a small minority of schools don't make the policy right," said deputy general secretary Martin Johnson.
"The academies policy leads to anarchy, breaking up the local education system, preventing sensible and efficient planning, and opening up free-floating schools to private firms with profit motives."
Asked on BBC News whether he was disappointed by union opposition to the academy programme and whether he thought it would hold it back, Mr Gove said this was not the case.
He accepted there would be different views in the teaching profession but said there were "people in senior positions in teaching unions" who were taking up the offer to change their schools to become academies.