Allowing schools to be run for profit has turned the Swedish education system into a "Wild West" that will be impossible to bring back under state control, according to the man expected to become the country's next prime minister.
Stefan Lofven, the leader of Sweden's opposition Social Democratic Workers' Party, used an interview with TES to warn other countries against fully implementing his nation's free school model and allowing operators to make profits.
His comments come after one of Sweden's biggest free school chains, JB Education, went bankrupt in May. The failure of the group, owned by Danish private equity firm Axcel, has intensified fears over the viability of the for-profit model.
The policy of handing control of schools to independent organisations has gained ground in England and the US, with charter schools in some American states run for profit.
Schools themselves cannot be run for profit in England, although a small number of companies have been handed contracts to run the management of schools for profit.
Mr Lofven said that countries should be "very cautious" about allowing for-profit schools to be established. "Look at what happened here. It has developed into something which is almost uncontrollable," he said. "We see today schools going bankrupt, so youth that had started their three-year upper-secondary school education find that after two years their school is gone.
"The school system has developed into the Wild West in Sweden. And it's not a good system."
Sweden's Social Democrats, who are leading national polls ahead of a general election next year, have been uncomfortable with free schools ever since they were introduced by a centre-Right government at the start of the 1990s. But the party has become increasingly critical as private equity companies have taken ownership of many operators.
Mr Lofven said that he believes free schools are partly to blame for Sweden's worsening performance in international educational league tables.
"School results are going down and the differences between schools are widening, so today Sweden is almost like Germany and the US when it comes to the importance of parents' school backgrounds in school results. We haven't been close to that before and it's not a good development."
A cross-party parliamentary committee set up in Sweden to investigate how to reform free schools proposed earlier this year that Sweden's schools inspectorate should have greater powers to monitor and license free school owners and that free schools should have transparent accounts.
However, Mr Lofven admitted that with more than a third of Swedish upper secondary schools and a sixth of elementary schools now run by private operators, it would be impossible to reverse the policy.
"Some of them are doing very well. It's not as if all private schools are doing (badly)," he said. "It's more important to deliver quality than it is who runs it. Quality is the most important thing."
Instead, the Social Democrats want to give the local municipalities that are in charge of Sweden's government-run schools more power over free schools, put in measures to force a longer-term approach and enshrine whistle-blowers' rights when things go wrong.
"If you want to run a school in Sweden you need to prove that you're long term and you need to show your books," Mr Lofven said. "If you have a school and you lower the number of teachers, for example, and make a profit, and send that to your bank account, you can do that today. So we're saying, 'No, that's not OK. If you're publicly financed, you need to show your books.'"