Schools should be allowed to teach children in classes of 70, the schools minister has said.
Despite concerns that pupils struggle in large groups and Labour pledges to reduce class sizes, Jim Knight said super-sized classes were "perfectly acceptable".
Mr Knight said lessons could be managed if up to four teachers or teaching assistants were there to help.
The Conservatives warned that such large classes would mean pupils with vastly different abilities would be more likely to be taught together, which would make it difficult to boost standards among poor pupils or push the brightest.
One delegate at the annual conference of the Association of Teachers and Lecturers (ATL) in Torquay complained that some primary schools were being forced to teach children in classes of 38.
Mr Knight said: "Class sizes are obviously something we take seriously. If they are growing to the extent that the delegate talks about then there are some concerns attached to that.
"Teaching assistants and higher level teaching assistants working alongside teachers are very important to ensuring that class sizes of 38 are manageable."
But teachers in the 400-strong audience jeered. Phil Jacques, the ATL's executive member for Dorset, said: "Class sizes of 38 should not be made to be manageable. They simply shouldn't exist."
Michael Gove, the shadow children's secretary, said: "The Government cannot simultaneously say it is going to deliver personalised learning and then support class sizes at the level Jim Knight is talking about.
"We have seen a trend over the last few years towards bigger classes and bigger schools. That runs directly counter to parents' priorities and is not the right direction for education in this country."
By law, infant classes must have a maximum of 30 children. Despite a pledge to eradicate illegally large lessons, 23,250 pupils aged five to seven remained in large classes in 2006. Ministers do not prescribe class sizes for older children.
Speaking after the conference, Mr Knight said classes of more than 50 pupils were acceptable. He quoted the example of Thomas Telford School, in Shropshire, in which 70 pupils share a mathematics lesson.
"What I saw was perfectly acceptable. There was good learning going on," he said.
Later, he added: "It is ridiculous to suggest we want a single teacher to teach a class of 70. In fact, over the last 10 years we have significantly reduced the ratio of pupils taught by adults in primary and secondary schools.
"In the case I was discussing, four teachers taught a class of 70 offering more, not less, individual attention."
In 2006 5,755 secondary school children were taught in classes of more than 51.
