Classroom assistants are threatening a national pay strike, saying schools are using them as “teachers on the cheap”.

Jim Knight, schools minister, was accused of sacrificing teaching assistants’ pay and conditions when he attended a support staff union conference in London this week.

Unison, which represents more than 200,000 support staff andclassroom assistants, agreed to ballot for a one-day national strike in July over a pay offer of 2.45 per cent.

Assistants say they are expected to do the job of teachers, but starting pay under pound;15,000 compares unfavourably to teachers’ pound;20,133.

Their pay increase is the same as that offered to teachers this year, which was accepted by most unions. But the 2.45 per cent pay rise sparked a national strike by the National Union of Teachers last month.

The NUT is to discuss further industrial action next week and one option is walking out on the same day as Unison, but the teachers are likely to hold off until September.

Across England, there are about 500,000 teaching assistants and other support staff, outnumbering teachers. When they went on strike in Birmingham in February, they shut 168 schools.

Mr Knight announced to the Unison conference that a new pay negotiating body for school support staff would be set up by September, with an independent chair. Unions and school employers will sit across the table from each other, hammering out national pay grades for support staff.

But that exacerbated the concerns of some teaching assistants, who fear losing the clout of joint negotiations with other local government workers.

Conference delegate Mike Forster, from Kirklees, told the minister: “Many of our members feel that schools have got teaching on the cheap, rather than paying the rewards they should.”

And Vicky Perrin, a primary teaching assistant from Calderdale, said schools were taking advantage of teaching assistants, and the negotiating body would be toothless to remedy that.

In response, Mr Knight said that the negotiating body would protect support staff pay and conditions, with representatives of employers from all kinds of schools except academies and independents.

The Association of Teachers and Lecturers, which also represents assistants and other staff, welcomed Government recognition that support staff were professionals.

Mary Bousted, the association’s general secretary, said: “For far too long many support staff could earn more stacking shelves in a supermarket than supporting a child with special educational needs.”