The National Union of Teachers (NUT) is to vote on a proposal to limit classroom sizes in England and Wales.
Teachers may strike if a limit is not put on class sizes
Schools Minister Jim Knight provoked teachers' anger this week when he said classes of 38 and even 70 pupils could be managed with teaching assistants.
The NUT's annual conference will vote on whether to demand limiting class sizes to 20 by the year 2020.
There is a call for industrial action if Westminster and the Welsh Assembly refuse to implement the policy.
Head teacher Chris Hole told the BBC the kind of class sizes considered acceptable by Mr Knight would not work.
He said: "Seventy in a class has a physical problem, but also in terms of educational development and deep-seated learning, it's something that is not really advisable."
But Mr Knight told the BBC he had not recommended larger class sizes, but had only said they were possible if the teaching was of a sufficiently high standard.
He said: "I never said I wanted children to be taught in classes of 70, or that I wanted classes of 70 taught as a general rule in schools.
"What I was saying was that it's not just about class sizes, it's also about the quality of the teaching."
The NUT's general secretary Steve Sinnott told BBC News 24 that falling admissions should allow class sizes to be reduced.
He said: "We are going to go through a period of falling roles.
"What we want to ensure is we don't get rid of teachers or reduce the number of teachers in the education system as those falling roles work their way through the schools."
He said they should instead use the period to reduce class sizes and give youngsters more individual attention.
The union has also come into conflict with the government over pay.
The conference in Manchester will ballot members over whether to take a one-day strike on the issue, scheduled for the third week in April if members vote yes.