Primary school teachers should ideally have no more than 20 pupils in their class, according to the National Union of Teachers (NUT).
Christine Blower, general secretary of the NUT, said it is not acceptable that classes in the UK are the fullest in Europe.
Figures published by the Organisation for Economic Co-operation and Development show that British primary school teachers have an average of 25.7 pupils in their class.
Schools in Austria, the Czech Republic, Denmark, Finland, Greece and Iceland have class sizes of fewer than 20 on average.
"The NUT has for a number of years called for class sizes to 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," Ms Blower stated.
She noted that small class sizes lead to more motivated and better educated youngsters.
Having fewer children to teach means teachers can dedicate much more time to each pupil, Ms Blower added.