David Walker, chairman of the Professional Association of Teachers, will attack the other three classroom teacher unions who have all balloted their members this term about action over class sizes.
Mr Walker, a former head who is now a supply teacher dealing with disaffected children, will say: "Some of the people currently making the running about class size are more concerned about their own welfare than the children's."
He adds: "A group of 40 sensible, well-behaved children who know what to do and are willing to do it is quite manageable."
The association, which never strikes, is opposed to a legal limit on class size. Mr Walker says: "The truth is that the issue of class size is far more complex than is being admitted at the moment. It isn't automatically true that smaller classes mean better teaching and learning.
"Some things really do need to be taught in small classes: one example is the teaching of reading. Another is practical subjects, especially where safety is an issue.
"Other subjects, however, lend themselves quite happily to larger class sizes. Indeed pupils can benefit from the kind of group discussion only possible in a large group."
By contrast, a class of 12 could be a nightmare. Given the variables, Mr Walker believes, there should be no arbitrary limit on class size as there is in Scotland and many European countries.
Jackie Miller, the association's deputy general secretary, said the danger with a legal limit was that it might become the minimum.
A spokesman for the Association of Teachers and Lecturers, which is opposed to classes of more than 31, said: "Our members would be unanimous in saying that Mr Walker is wrong. You will never find a private school advertising the fact that it has a class of 40."Reuse content