The schools minister who believes supersize classes of 70 pupils are acceptable

In noisy scenes, Jim Knight was greeted with cries of "No!" as he insisted primary classes of 38 were manageable.

The schools minister went on to claim that groups of more than 70 were "perfectly acceptable" if teachers were helped by classroom aides.

A leading member of the Association of Teachers and Lecturers gave him a lengthy public dressingdown following his keynote speech at their annual conference yesterday.

It was among the most critical receptions a minister has received from the most moderate of the main three teaching unions.

It follows a deal ministers struck with unions to reduce staff workloads with greater use of classroom assistants.

Tens of thousands have been deployed, but qualified teachers are warning that the helpers are being used to justify supersize classes as schools increasingly put two or even three classes together.

Figures obtained by the Tories last year suggested that almost 6,000 pupils were being taught in English or maths classes of more than 50, with 715 in groups of more than 60.

In contrast, independent schools are investing heavily in teaching staff in an attempt to appeal to parents.

The row was sparked by a question-to Mr Knight from Malcolm St John Smith, a delegate from Wakefield, who demanded to know how teachers could be expected to lead a class of 38 children as young as seven with "limited assistance".

Mr Knight told the conference in Torquay that assistants could help make large classes "manageable".

"Class sizes are obviously something we take seriously," Mr Knight said.

"Teaching assistants and higher level teaching assistants working alongside teachers are very important-to ensuring that class sizes of 38 are manageable."

But later, to cheering and clapping by delegates, Phil Jacques, ATL's executive member for Dorset, declared: "Class sizes of 38 should not be made to be manageable. They simply shouldn't exist."

Afterwards Mr Knight said he had visited a school in Telford where a "charismatic" maths teacher was giving a lesson to more than 70 students, helped by big screens and teaching assistants.

"There was good learning going on," he said.