A new study has revealed that most 14 to 16-year-olds are being taught key subjects by unqualified teachers.
In mathematics and English, on average, more than half of all teachers either do not have an accredited teacher-training certificate or the competence to teach their subject.
In some parts of the country, such as Tingsryd and Morbylanga, more than 80 per cent do not have the required qualifications.
The study, conducted by Statistics Sweden, was commissioned by two teachers' unions after doubts arose about the validity of the Swedish National Agency for Education's claim that 84 per cent of all teachers working in state schools are fully qualified. The government-funded agency study did not take into account those teachers who taught subjects outside their area of competence.
The biggest shortcomings are to be found in sciences and maths with 56 per cent of teachers fully qualified compared to 65 per cent working in the humanities and social sciences.
The study also reveals that Sweden's state-funded independent schools - which inspired Tony Blair's trust school proposals - are not doing enough to recruit qualified staff.
On average almost one in three teachers in the independent sector lack teacher training or formal education in the subject they teach.
Eva-Lis Preisz, chair of the Swedish Teachers' Union, and Metta Fjelkner, chair of the National Union of Teachers, said: "Our European colleagues will be appalled. Both state and independent schools are systematically breaking the law."
Ms Preisz and Ms Fjelkner are calling for tighter legislation and for inspectors to take immediate action to safeguard the interests of pupils.
In comparative tests in 2003, Sweden came fifth in literacy, but its pupils performed just above the Organisation for Economic Co-operation and Development average in maths, coming 10th, and the country fell to 14th in science.