At least a further 500 free schools would be opened in England in the next five years under a Conservative government, David Cameron has pledged.
He committed his party to creating an extra 270,000 school places in free schools, if re-elected, by 2020.
The prime minister says these state-funded, start-up schools are “raising standards and restoring discipline”.
Shadow Education Secretary Tristram Hunt says free schools lead to school places where they are not needed.
Speaking in London, Mr Cameron announced approvals for 49 more free schools and promised a “dramatic” expansion if re-elected in May’s general election.
Free schools, both primary and secondary, are set up by academy sponsors, charities, teachers and groups of parents, and operate outside local authority control. Academy sponsors include academy trusts, faith groups, universities and companies.
Mr Cameron described free schools as “independent schools within the state sector” – and with those announced on Monday there will be more than 400 open or approved, with 230,000 places.
If Conservative plans were to be implemented in the next Parliament, there would be 900 free schools, more than one in 30 state schools, with about 500,000 places.
By Hannah Richardson, education reporter
Arguably one of the most controversial of the coalition’s education policies, the free school programme is much more than giving parents and teachers the right to set up their own schools.
The complexities of running a school mean many of these new state-funded, but independently-run, schools are actually led by academy chains and education companies.
And the crucial rule that any new school, even in school-place squeeze areas, has to be a free school or academy, has led to an impasse in school-place planning in some districts.
Indeed, some local councils have struggled to find organisations to set up schools where they are needed.
The government cites figures suggesting 70% of inspected free schools are rated good or outstanding.
And supporters argue free schools offer choice and quality, as well as new school places, and drive up standards in other schools as a result.
But with a handful of high-profile cases of free schools failing within months of opening, question marks have been left hanging over these claims.
Mr Cameron said that the Conservatives are the “only party that’s opening up the education system so we can get more good places for your children”.
He said free schools “are not only outperforming other schools, but they are raising the performance of those around them, meaning more opportunities for children to learn the skills they need to get on in life”.