Teachers are planning a campaign of industrial action in a bid to force ministers to limit class sizes to a maximum of 20 in state schools.
The National Union of Teachers (NUT) condemned assertions this week from a minister that classes of 70 could be "perfectly acceptable".
Delegates at the NUT's annual conference in Manchester this weekend will vote on whether to launch a campaign to limit class sizes to 20 by 2020.
The union's general secretary Steve Sinnott said: "I think a campaign to reduce the size of classes in the state sector to the level of the independent sector would be a very popular campaign with parents indeed.
"There is a need for us to explain what we are doing to parents and for parents to make clear their support for reduced class sizes to Government.
"We want to have a strategy that looks at involving people with our objectives, negotiating with Government and where it's appropriate using industrial action too."
Speaking at the Association of Teachers and Lecturers (ATL) conference earlier this week, Schools Minister Jim Knight said he had seen a maths class of 70 pupils working well in a large room with one teacher and three or four other adults.
It was not clear whether these other adults were teaching assistants or fully qualified teachers.
But the NUT said pupils need a qualified teacher to make sure they experience "quality learning".
Mr Sinnott said: "I think the argument that Jim Knight is talking about is an unacceptable argument. I'm not surprised that there was an outcry on the part of teachers at the ATL conference."
