Class sizes should be limited to a maximum of 20 pupils by 2020 to improve the academic achievement of pupils and give teachers a better work life balance, school leaders are demanding.

The National Union of Teachers (NUT) will this weekend argue at their annual conference in Manchester that the move will also improve discipline and enable teachers to give more personalised attention to individual pupils.

The debate follows suggestions earlier this week from the schools minister Jim Knight that class sizes of up to 70 pupils were perfectly acceptable and could even promote good learning, providing classroom aides are also present.

Class sizes in England are among the largest in the developing world, with an average of 26.2 children in every primary class and 21.1 in secondary classes in the state sector.

The UK comes 23rd out of 30 countries in the 2007 OECD survey of class sizes and more than 14 per cent of primary children and 11 per cent of secondary children are taught in classes of 31 or more in the state sector.

With with an average of only 10.7 children per class in independent schools, the gap in class size between private and state sector is higher in the UK than anywhere else in the developed world.

Steve Sinnott, the union's general secretary, called for smaller class sizes to be phased in with immediate effect.

"Finland, which is considered the highest achieving country in Europe, enjoys a maximum class size of 20, while it is anticipated that in Scotland class sizes for the first three years of primary education are to be reduced to a maximum of 18," he said.

With school rolls currently falling following a dip in the birth rate, proposers of the motion at the NUT believe that now is the perfect time to introduce a maximum limit on class sizes.

They are calling on the Government and local authorities to reconsider plans to close down under subscribed schools, suggesting instead that they be reorganised with smaller classes.

Although the Government has introduced a strict limit of no more than 30 pupils in primary schools for children in Key Stage 1 (for children aged five to seven), schools are not always able to accommodate this.

For children older than seven, ministers have been unwilling to legislate for a maximum class size beyond this, despite research linking high attainment to class size.

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