The National Association of Head Teachers should abandon its policy of recommending that heads should refuse to put a figure on class sizes, and instead call for an average of 25 in each class, Mick Brookes, chair of the union's primary sector committee said this week.
He warned the union's annual primary conference in Bristol that the Government was cynically using the reluctance to talk numbers against the union by claiming that heads and governors wanted to be free to manage.
The union also announced plans for a three-year study to follow up its recently published research on class sizes. It hopes the study will add further weight to the argument that smaller classes are best.
Mr Brookes said: "The Department for Education and Employment is using our rhetoric against us. We have to be specific and say an average class size of 25, with maximum limit of 30."
Professor Chris Day at the School of Education, University of Nottingham, will be responsible for the new study. He produced the recent class-size research and its effect on the quality of teaching.
That report, published last month, disputed claims cited in a report by the Office for Standards in Education and widely used by ministers that class size does not matter.
The new project, which still needs final funding, will involve looking at 48 classrooms in 24 primary schools grouped into those with fewer than 30 pupils, more than 30 pupils and more than 40 pupils. It will look over time at the effect on teachers' planning and teaching and children's learning experiences.
The teachers taking part will be chosen for their competence levels to pre-empt any claims that the difference in achievements was due to the standards of teaching rather than the class size.
Professor Day said: "As well as looking at pupils' achievements in classes of different sizes we hope to take a more holistic approach on what effect class sizes have on children and teachers.
"What it should do is provide an empirical information base which is sadly lacking at the moment and which will inform policy-makers."
David Hart, the union's general secretary, highlighted the subject in his keynote address when he described the class-size issue as "a major scandal".
Average class sizes had risen year by year, he said, with nearly 80,000 primary children now being taught in classes of 37 or more. This was an increase of 13,000 in one year.
The NAHT is also planning a national development plan for primary schools to set out the curriculum and resources which would give children the best deal.
This follows research by the primary sector committee which shows that key stages 1 and 2 are grossly underfunded Mick Brookes said: "I think it is very likely that many of the problems at secondary level could be alleviated if there was more money at primary."