Sweden has had the free school system that the UK Conservative party are advocating for some 15 years now. And during this time a number of serious problems have become evident that mean urgent reform is now necessary. In fact, it is exactly those parts of the system the Tories want to implement in Britain that we are proposing to put an end to in Sweden.

The reforms will not work without extra investment. The Labour party and the Swedish Social Democrats propose rising schools spending when the Conservatives in the UK and Sweden propose less. Spending alone won't always improve standards, but creating surplus places like this without providing the funding to allow for the surplus capacity you need could seriously harm standards.

A country's future lies in how well we educate and take care of our children. Every parent knows that special blend of excitement, pride and worry that you feel when your child goes to school for the first time. How will it go? Will they make friends? Will there be a teacher who sees the potential within every child?

Yet the Swedish authorities' own research has concluded that over the last fifteen years since the free schools were introduced, the number of low performing pupils has increased in Sweden, while the high performing pupils have neither increased in numbers nor have they become more successful.

That is why it is worrisome when the Tories want to copy our system by picking out the bad apples of the basket.

The free school system, implemented without imposing clear standards, has seen schools opening with sub-standard facilities, often without libraries, and with a far greater number of unqualified teachers.

What's more, the introduction of free schools has led to increased segregation where pupils from the same social background increasingly concentrate in certain attractive free schools.

This matters because segregation and poorer facilities serve no-one but the Conservatives seem to specifically think that these "freedoms" are positive aspects of the policy. This is a serious mistake.

To some extent, there is an irony in the fact that the British Tories are looking towards Sweden as an example for educational policies, when at the same time Swedish politicians – progressives as well as liberals and conservatives – are finding answers to some of our challenges in Britain. I am not only thinking about the British universities, but also the primary school system. We are deeply impressed by the one-to-one tuition and catch-up support, but also how you have been able to raise attraction to society's most important profession: the teacher, by the Teach first-program, which now is investigated and advocated both by us in the red-green opposition and by the conservative government. These and other Labour-initiated programs serves as examples for us.

If we win the Swedish general election in September, we won't prevent parents from choosing free schools for their children. But we will reform the system in order to reverse the serious problems that have become evident over in this system, increasing spending on schools. Spending alone won't always improve standards, but creating a free market as the Conservative proposals do without providing the funding to allow for the surplus capacity you need will certainly harm standards.

I sincerely hope there are aspects of the Swedish school system – especially how a system aiming at cohesion and equality in the system raises the performing results – that you can learn from us. But implement our successes – do not repeat our mistakes.

• Mona Sahlin is leader of the Swedish Social Democratic Party