Almost a third of free schools have employed non-qualified teachers, according to government research which claimed that the newly created schools were using their freedoms to be more innovative.
The figures show that 32 per cent of free schools surveyed had employed teachers with no formal qualifications.
The numbers will cause concern to both Labour and the major teaching unions, which have called for all schools to employ only staff who have qualified teacher status (QTS) or are working towards gaining QTS.
Education secretary Nicky Morgan announced a further 35 free schools at the Conservative Party conference yesterday, meaning that there are now more than 360 free schools either open or approved to open. The schools have always been exempt from requirements to employ teaching staff with formal qualifications.
In her speech in Birmingham, Ms Morgan said free schools were "outperforming other schools and many are pioneering new ideas and approaches to give pupils and parents greater choice and opportunity".
The research shows that of the 32 per cent of schools that had employed non-QTS staff in the past, 46 per cent either no longer employed non-qualified staff or had just one non-qualified teacher.
The government gave free schools the freedom to employ non-qualified teachers to enable them to bring in subject specialists, as many independent schools do. The policy was then expanded in 2012 to include academies.
Shadow education secretary Tristram Hunt has pledged that, under a Labour government, a qualified teacher would work in every classroom. He has also called on prime minister David Cameron to “scrap” the policy that Labour claims has produced a 16 per cent increase in the number of unqualified teachers in UK schools.
"The surest way to improve the learning outcomes for our children is to deliver highly qualified, highly motivated teachers. That is where my focus is," he said in a statement to TES.
“My ambition is for a world-class teacher in every classroom. Under Labour, all teachers will have to become qualified, as a bottom line. We will offer new high-status career routes and better training and development – all conditional to remaining in the classroom.”
The survey contacted headteachers at the 174 free schools that were open during the 2013-14 academic year, of which 74 responded.
It revealed that 72 per cent of heads thought their free school was having a wider impact on schools in their area and a third felt that they had helped to raise standards through competition.
Furthermore, 84 per cent said they were working in a formal collaboration with their neighbouring schools, or were intended to start working closely with them.
Unions step up attacks on use of unqualified teachers - 19 April 2014
Labour uses Pisa results to attack government support for unqualified teachers - 3 December 2013