The biggest teaching union is refusing to sign an agreement with the government on the increased use of teaching assistants which it says could mean a "mums' army" in classrooms.

An agreement covering England and Wales, following lengthy negotiations, is due to be signed by 15 January.

But the National Union of Teachers is demanding a limit on the number of assistants allowed to take classes.

The government has promised to recruit an extra 50,000 classroom assistants before the next election and insists they are not for staffing schools on the cheap.

Ministers want to use them in their plan to "re-model" the teaching profession and reduce teachers' workloads - cited as the biggest turn-off by teachers leaving the profession.

Protection

The NUT's general secretary, Doug McAvoy, said: "If the government introduces these proposals without any checks or balances, it would be possible to change the balance between qualified and unqualified teachers in any school.

"We beat off the idea of a mum's army of classroom helpers under the Tories.

"These proposals could take us down the same road unless regulations protect the number of qualified staff."

He said he did not believe ministerial assurances that they would do no such thing.

"I don't trust them," he said.

"Not because I think they're devious - I think the temptation to cut back on the cost of staffing schools by employing the cheaper person, that is the teacher assistant rather than the teacher, will be great for this government."

Call for more pay

The general secretary of the National Association of Head Teachers, David Hart, said: "I think it would be absolutely crackers for any organisation that represents primary school teachers to refuse to sign up for this agreement.

"Most of their members are in primary schools and they are the ones who stand to benefit the most from it."

At present, primary school teachers had no guarantee of any time away from the classroom. The government's proposed deal would give them that.

The senior national education officer at the public service union Unison, Christina McAnea, told a seminar on the subject of assistants at the North of England education conference, in Warrington, that 30% of classroom assistants already took classes.

A fifth covered for absent teachers and Unison believed they deserved better pay and training for the extra responsibilities they were being asked to take on.